I am a sucker for interesting, but usually useless information of military tradition and such - often most useful in bar bets. I used to have a whole bucketload of stuff like this, but unfortunately I never wrote it down, kept it all in my head, and some evidently seeped out. All of us have so much of this type of knowledge from our varied service and experiences; I think it would be fun to share. Mine was mostly Naval- or Marine-related, but I'll start off with a few I've retained. Some may be familiar to some:
Leatherneck - A Marine, from the leather stock worn around the neck in the early 1800's uniform, supposedly to protect from sword cuts. Kept the head erect, too.
Tar - a sailor, who used tar on his braided hair (pigtail) to keep it from coming loose, possibly getting caught in rigging. Also the reason why the traditional sailor jumper uniform (crackerjack) has the wide collar on the back (originally a separate piece) - to keep the tar from staining the shoulders and back of the uniform. Originally (up till the 1920's) even the "summer white" jumper had a black/navy blue collar so the tar would not show.
Binnacle List - List of personnel not fit for duty, or "Sick List". Name comes from the list, usually compiled by the Ship's Cook before watch. He would write down the names of the sick, and place it in the ship's binnacle, which housed the ship's compass. The binnacle was watertight, and located near the helm on the quarterdeck, the Captain's "office" on deck. There, the list would be protected from the weather, and available to the Captain when he first came up on deck at beginning of watch.
"Shake (or Show) a Leg" - In the old days of the Royal Navy, some men were allowed to take wives along, or, in other cases, would bring aboard a "wife" for the evening from shore leave. At the beginning of watch, the Ship's Cook, or Bo'sun would go through the berthing area waking the on-coming watch for duty. He would often call out "Show a leg!" so the man would stick his leg out of his bunk or hammock, and the man performing the wake-up call would not cause a faux pas by grabbing a shapely, feminine one.
Grog - another holdover from the Royal Navy. Water did not keep well on long sailing voyages, and in 1740, VAdm. Vernon (nicknamed "Old Grog", due to his wearing a cloak made of "grogram" [grosgrain?]) instituted the issuance of rum added to the water ration of British sailors. Sugar, lime juice (hence the nickname "limeys"), molasses, and other ingredients were added. Over time the distribution of the rum ration became encrusted with elaborate ritual. At 11am the Bo'sun's Mate piped 'Up Spirits,' the signal for the Petty Officer of the Day (POD) to climb to the quarterdeck and collect (1) the keys to the spirit room from an officer, (2) the ship's cooper, and (3) a detachment of Royal Marines. In procession, they unlocked the door of the spirit room, and witnessed the pumping into a keg of one eighth pint of rum for every rating and petty officer on the ship aged 20 or more and not under punishment. Two Marines lifted the keg to the deck, standing guard while a file of cooks from the petty officers' messes held out their jugs. The Sergeant of Marines poured the ration under direction of the Chief Steward, who announced the number of drinking men present in each petty officer's mess. The rest of the rum was mixed in a tub with two parts water, becoming the grog provided to the ratings. At noon the bo'sun's mate piped "Muster for Rum", and the cooks from each mess presented with tin buckets. The Sergeant of Marines ladled out the authorized number of “tots” (half-pints) supervised by the POD. The few tots of grog remaining in the tub ('plushers') were poured into the drains (“scuppers”). Sadly, this tradition ended in the US navy in 1862, and the last issue of grog in the RN ("Black Tot Day") was 7/31/1970.
scuttlebutt - Today, this refers to a drinking fountain/water cooler, and also gossip/rumor. The two definitions come from the same origin. A "butt" is a wooden cask or barrel, containing 2 hogsheads of liquid, or about 126 gallons. This was commonly used to store water aboard ship. To "scuttle" means to cut or bore a hole in a vessel. So to "scuttle a butt" meant to cut a hole in the barrel, to dispense water. Basically the same as "tap a keg". So when time came to issue the water ration to the crew, a butt would be scuttled, and the men would gather around to draw their ration, drink, and talk about last night's shore leave, complain about the Chief, where's their next destination, start rumors, conspire to mutiny... This "scuttlebutt talk" became what we call today "water-cooler talk."
Splice the Main Brace - an offiically-santioned, celebratory issue of liquor or grog to the crew, for a special "job well done", usually a battle victory or getting through a dangerous situation. The "main brace" was part of the rigging of a sailing vessel which carried the greatest amount of strain, and held the masts in place. It was a thick rope, or hawser, about 4" in diameter, and very difficult to work with. Being such an important part of the rigging (the ship would be disabled without it), it was often the target of enemy gunners who wished to capture, rather than sink a ship. If the main brace were severed in battle, the best and ablest seamen would be sent aloft, to "splice the main brace". They would perform this difficult task, in the thick of battle; exposed to shot, shell and Marine snipers, while suspended above a tossing and pitching deck, and unforgiving ocean. Those who successfully performed the repairs and survived, were given a reward of extra grog, or a shot of whiskey. This tradition still occasionally happens; shot-down Navy pilots are often given a "shot of medicinal brandy" after rescue at sea.
I'm rather embarassed that most of what I've written is Navy related and not Marine. Perhaps something someone else adds will jog my memory...