What is the meaning and significance of various military words and expressions including 'semperfy'? What are some others?

Semper Fi is shorthand for Semper Fidelis, which means always faithful. What else you want to know?

Semper Fidelis or Semper Fi, from the Latin means "always faithful." Motto of the U.S. Marine Corps.

Here's some Marine terms for ya...
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Deuce gear - web gear, from the 782 form
Bag/Box nasty - bagged lunch or WM in bed
WM - female Marine
Fart Sack - sleeping bag
Pork Chop Platoon - Physical conditioning platoon
Army gloves - trouser pockets
The Stumps - MCAGCC, 29 Palms, CA
Brain Housing Unit - your head
Dick skinner - your hands
Snot locker - nose
Broke dick - Marine on light duty
Brain bucket - helmet
Prick - radio
Donkey Dick - used to clean the mortar tube
Rat Fuck - take the best chow and leave the junk

Other military expressions would be:
"Gig": to be "gigged" is to have a notation made regarding something that is "out of line", or a deficiency of some type.
"Police": to clean up.
"UA": Unauthorized absence (same as Army's "AWOL")
"Irish Pennant": Marine Corps term for a loose thread on a uniform
"Piss Cutter": type of hat worn by servicemen
"Cover": Your hat, as in "remove your cover". (Marines)
"Snoop and Poop": Recon an area for enemy in a stealthy mode
"Recon": Infiltrate and investigate an area (usually enemy territory)
"Devil Dog": Marines, term given them by Germans in WWII
"Lock and Load": Load ammo into your rifle or weapon.
"Blanket Party": Unofficial punishment where an individual is ambushed in his bed at night, pinned down by his blanket, and beaten by his comrades for failures.
"Sat": Satisfactory.
"Unsat": Unsatisfactory
"Head": Restroom (navy, Marines)
"Rack": Bed
These are just a few. The military has many terms for various actions, equipment, and behaviors.
As for acronyms: the military is as full of acronyms as a chicken is covered in feathers.

What are some of the most commonly used acronyms and what do they mean?