Helpful and commonly used terms:
[color=f6989d][b]UA[/b][/color]:  Unauthorized absence.  This is an actual crime under military jurisdiction if you are not to you designated and/or assigned place at the assigned time, but it is often used casually, meaning you just don't know where a person is at.  "Where's Jon?"  "Uh, guess he went UA."
[b][color=f6989d]Weapons free/hot[/color][/b]:  This means that from this point forward, any body that you encounter not with you is an enemy and can be engaged immediately with lethal force.  This may seem like a soldier's wet dream, but there are 2 realities:  you are in a place where literally EVERYONE is hostile, meaning you are immersed in enemies, and second, you may come a point where you engage someone who is obviously not hostile towards you, but you must assess whether to engage with lethal force.  This may be a child, a man in a wheelchair, a pregnant woman.  Further more, in situations, there is nothing STOPPING that child, man in a wheelchair, or pregnant woman from BEING an actual hostile.
[b][color=f6989d]RP[/color][/b]:  Rendezvous point.  This is a designated place to meet with instructions.
[b][color=f6989d]HVT[/color][/b]:  High Value Target.  This generally refers to a person, but on occasion can be used to describe an object.
[b][color=f6989d]Can'c[/color][/b]:  Pronounced like "cane-k-d" and short for cancelled.  "Hey man, the brief was canc'd, let's go drink."
[b][color=f6989d]CAS[/color][/b]:  Pronounced "kah-z" or "kah-s" and meaning close air support.
[b][color=f6989d]Ma Deuce[/color][/b]:  Slang for MA-2, a fully automatic, 50. caliber, heavy machine gun.
[b][color=f6989d]Exfil[/color][/b]:  Exfiltration or extraction point.  "Send me the position for our exfil so we can leave!"
[b][color=f6989d]Click[/color][/b]:  One kilometer.  "We're 3 clicks from our target."
[b][color=f6989d]Mic[/color][/b]:  Pronounced "Mike," meaning minute.  "I'm getting my gear, gimme two mics."
[b][color=f6989d]Top[/color][/b]:  First Sergeant, generally in a Marine or Army unit.  "Hide the liquor, Top's coming!"
[b][color=f6989d]Butter Bar[/color][/b]:  A derogatory term for a fresh Lieutenant.  Derived from their rank insignia being a bronze bar, looking like a stick of butter.  "We're lost because Butter Bar over there can't read a damn map."
[b][color=f6989d]ROE[/color][/b]:  Rules of engagement.  This is the rules for what types of force are allowed and when. 
[b][color=f6989d]Ninja Punch[/color][/b]:  Slang for Non-judicial punishment.  This is for offenses handled by the military via non-Court Martial.  "Dude, did you hear about Rodriguez.  He got hit with the ninja punch for showing up drunk to PT!"
[b][color=f6989d]Frag[/color][/b]: Either referring to an M67 hand grenade ("Dude, toss a frag!") or telling someone to utilize an M67 ("Corporal, frag that room, now!").
[b][color=f6989d]Bang[/color][/b]:  Similar to FRAG, it references a flashbang grenade (also stated as BANG).
[b][color=f6989d]Danger Close[/color][/b]:  This is generally used in 2 instances.  The first is when ordinance or indirect fire is being used at a distance that is not normally advised--so much so it may injure the individuals it is intended to help.  The other instance is stating that hostile enemies are in an EXTREMELY close proximity, meaning perhaps the other side of a tree a person is taking cover behind.