I thought these words were interesting and their original meanings were often surprising . I'll post more as I find them.
AMBULANCE: A horsedrawn wagon that transferred wounded soldiers from the battlefield to a 'hospital', usually a tent or a barn. Not an ambulance as we think of it today. All roads were dirt at this time, and wagons had no shock absorbers, so these were very bumpy rides that often killed already badly injured men.
BUMMER: "A word with a similar meaning to 'deadbeat', used not only for a malingerer or loafer but also for a straggler or deserter. In the 1850s, it simply denoted a shiftless person, but it acquired its more nefarious meanings during the Civil War. Then it was used to describe a person safe in the rear, such as a cook or member of the medical staff, but it also was applied to a forager, a soldier who left the ranks and plundered, either alone or as part of a raiding force. Perhaps the most notorious were Sherman's bummers, stragglers who robbed civilians and vandalized property during Sherman's 1864 March to the Sea. Some were foraging to supply the Union army, but others were simply deserters taking advantage of the chaos produced when an army passes through a region." - historynet.com
"Came Through With Flying Colors" : In the Napoleonic style of warfare used in the late 1700s and early 1800s, the army's flags (colors) on the battlefield were strategically placed at the forefront of a battle line to inspire the soldiers and identify their regiments. They were of vital importance on the field; having one's colors captured by the enemy was a sure sign of defeat. They were not allowed to touch the ground, so if a carrier fell it was picked up immediately by someone else. Color-bearers carried no weapons, so to be one required incredible courage. Flags made easy targets and were often the first to fall under fire. So logically, to "come through with flying colors" meant to be victorious.
CONTRABAND(S): Escaped slaves who came across the Union lines for protection and freedom. To be found by invading Confederates meant almost certain recapture or death. Any family who harbored escaped slaves was an enemy of the South. In the movie "Glory", Robert Gould Shaw reads a proclamation sent to the Union Army stating that any negro found serving as a Federal soldier would be considered an escaped slave and put to death. Any white man commanding negro soldiers would be considered a slave rebellion inciter and also be executed. Since the war, "contraband" as an Army term has come to mean any items found on a person or in his or her belongings that are against rules, laws or regulations.
DEADLINE: Originated in the Andersonville prison camp. Described in a soldier's log from 1864 as a perimeter about fifteen feet from the inside walls of the fort, which any man who crossed would be shot on sight. Since then it has been absorbed into the language and come to mean an absolute time limit.
DEADBEAT: A loafer, a work-shirker. An intensification of the term "beat", used for any soldier who intentionally avoids his duty in camp.
DIXIE: refers to the Mason-Dixon line: A boundary surveyed in the 1760s that ran between Pennsylvania to the North and Delaware, Maryland and (West) Virginia to the South. It became a symbolic division between free states and slave states. "Dixie" is still a term for the South. "Dixieland" was a popular Confederate marching song.
DOG TENT: A two-man tent formed by two "shelter halves." Soldiers were issued half a tent in their knapsacks, they were required to buddy up with someone to button the halves together and form a tent. A shelter half by itself could offer some protection from the sun on a hot day but little else. The term may have come from disgruntled soldiers who said that these tents were only fit for dogs. Dog tents are still in use in today, and sometimes "pup tent" is sued to mean the same thing.
FASCINE: (pronounced fah-seen) A tightly bound bundle of straight sticks used to reinforce earthworks. Comes from the much older ancient Roman symbol of political power and strength, the 'Fasces', shown as a bundle of sticks around an axe handle. Around the time of the 2nd World War the term "Fascist" came into being as a style of government dictating absolute power from one individual. Interestingly, Benito Mussolini used the Roman Fasces as his insignia. If you look closely, the Fasces can still be found incorporated into medals and awards and insignia in our own military. It has become a universal symbol of power. It is another example of a symbol adopted for martial use which once may have had a different meaning, like a swastika.
"FLASH IN THE PAN" : Actually predates the Civil War and dates to the era of flint-lock muskets. Flint-lock firing mechanisms have a shallow "pan" that a small amount of powder must be poured on to be ignited by a spark from a spring-loaded, flint-tipped hammer, discharging the weapon. "A Flash in the Pan" could be when the powder in the pan goes off but fails to fire the weapon. The modern adaptation means any idea or plan which is very short-lived and a failure, something that never gets off the ground. (I used to think it was a cooking term before I found this out)
FORAGE: To search for whatever food is available. This was a Civil War term that in many cases was synonymous with theft. Soldiers often raided nearby farms for their pigs, cows, mules, chickens and their fruits and vegetables, appropriating them to feed the troops. "Forage Caps" were floppy hats worn by soldiers that theoretically could be used to hold small fruits or nuts when soldiers went foraging.
"Goober Pea": A common Southern term for a peanut. "Peas, peas, peas, peas, we're eatin' goober peas" went a popular marching song of the Civil War.
"IN GOD WE TRUST" - first authorized by the United States Congress in 1864 for a two-cent bronze coin. The coin is long gone but the slogan survives on two present-day coins, the nickel and the penny. - historynet.com
"HALF-COCKED" - "Half cocked" was the safety position on a rifle musket. The hammer was pulled back only halfway and could not fire from this position. If a gun fired from half-cock it had a defective lock spring and was dangerous. In modern times the term "to go off half-cocked" means any premature, poorly executed plan that doesn't work.
HOUSEWIFE: Small sewing kit soldiers used to repair their garments. Obviously a replacement for a real housewife in the field.
JEANS: A twilled cotton cloth. Actually a "jean" can be any material. The definition depends on the weave (always a 2/1 twill), not the material. CW period jean was most commonly wool on cotton, a cotton warp. "Jeans" meant clothing from "jean". Now the term "Blue Jeans" has evolved from this, cotton pants woven in a twill pattern.
"Infernal Machine": A term of contempt for torpedoes (either the land or the water variety). This term was also used to describe the Confederate vessel H.L. Hunley - the first successful submarine. This term has evolved to mean any devilishly complicated contraption that is prone to failure.
"Lock, Stock and Barrel": All parts of a rifle. Thus the term today came to mean "the whole thing".
MONITOR: Originally, the U.S.S. Monitor, the first ironclad warship in the United States Navy, commanded by Admiral John L. Worden. The vessel had a large, round gun turret on top of a flat raft-like bottom, which caused some spectators to describe it as a "cheesebox on a raft". The first engagement between ironclads occurred on March 8-9, 1862, at the Battle of Hampton Roads, VA, when the U.S.S. Monitor fought the C.S.S. Virginia (formerly the U.S.S. Merrimack). Eventually a "monitor" became the official term for an entire class of warships modeled after the original U.S.S. Monitor.
MORTAR: An unrifled artillery gun which was designed to launch shells over walls and enemy fortifications. The most famous Civil War mortar is the "Dictator" -- a mortar which was mounted on a railroad car and used during the siege of Petersburg. With its 13 inch bore it was capable of launching two hundred pound shells. Due to weak metal casting, mortar barrels had to be very thick to withstand the stress of firing. During World War II the term "mortar" came into use again but referred to a much smaller, portable infantry weapon. The WWII equivalent of a "Mortar" as it was known in the Civil War is called a Howitzer.
PAROLE: A pledge by a prisoner of war or a defeated soldier not to bear arms. When prisoners were returned to their own side during the War (in exchange for men their side had captured) the parole was no longer in effect and they were allowed to pick up their weapons and fight. When the South lost the War and the Confederate armies gave their parole they promised never to bear weapons against the Union again. "Parole" has since come to mean a length of time out of prison for good behavior.
PICKET: Soldiers posted on guard ahead of a main force. Pickets included about 40 or 50 men each. Several pickets would form a rough line in front of the main army's camp. In case of enemy attack, the pickets usually would have time to warn the rest of the force. "Picket" is still in use today to mean a line of public protesters, or to protest with signs.
PONCHO: A blanket or rubberized blanket made with a slit in the middle so as to be worn as a cape. Soldiers were issued these blankets that were canvas on one side and painted with rubberized tar on the other. Also known as "tar blanket" or "gum blanket". Now the term means any garment worn to protect from rain.
REVOLVER: A handheld firearm with a chamber to hold multiple bullets (usually 6). The chamber turns so that each bullet can be fired in succession without reloading. A term still in use today for such a weapon that did not exist before the 1850s.
Rifle-Musket: The common weapon of the Civil War infantryman, it was a firearm fired from the shoulder. It differed from a regular musket by the grooves (called rifling) cut into the inside of the barrel. When the exploding powder thrusts the bullet forward, the grooves in the barrel make it spin, just like a football spirals through the air. Rifle-muskets were more accurate and had a longer range than smoothbore weapons. "Rifle" has been adopted for modern use as a term for any weapon with a long barrel, and reserved for small arms, but in the Civil War a "Rifle" could describe a cannon with rifling as well.
SHEBANG: (pronounced sheh-bang) The crude shelters Civil War prisoners of war built to protect themselves from the sun and rain. "The Whole Shebang" now means the whole thing.
SHODDY: Term for cheap, poorly made cloth which was used early in the war to make Federal uniforms. The cloth fell apart very quickly. Eventually "shoddy" became a term for very inferior quality.
SHELL: A hollow projectile, shot from a cannon; a shell was filled with powder and lit by a fuse when it was fired. Shells exploded when their fuse burned down to the level of the powder. Depending on the length of the fuse, artillerymen could decide when they wanted the shell to burst. The term "shelling" meant an artillery bombardment with shells. Still used to mean an artillery round
TORPEDOES: Today called mines, Civil War torpedoes were mostly used by the Confederates. Sometimes they were buried in the ground in the enemy's path to explode when stepped on. Mostly they were used as water defenses. They floated below the surface of the water and exploded when the hull of a ship brushed against them. Today the term "Torpedo" refers to a self-propelled water projectile launched by a submarine. The phrase "Damn the torpedoes, full speed ahead!!" is attributed to Admiral Farragut, a naval officer.
YANKEE: A Northerner; someone loyal to the Federal government of the United States. Also, Union, Federal, or Northern. Now a derogatory term in the South. Originated during the American Revolution with the song "Yankee Doodle" or possibly much earlier, from the French Janqui.
ZIGZAG: trenches run out from parallels of attack, forming right angles by which the attackers could approach the enemy's lines. Right angles are used because they are easily defensible. The modern term "zigzag" now describes any path that takes that distinctive shape.