Full Metal Jacket It is a movie, but as that has been covered I will give it's original meaning.
FMJ is Full metal jacket. A bullet that has a copper (or some other metal) covering or "jacket" over its lead core. This is done so that the bullet has more penetration and won't pancake on impact. Note some bullets have no metal jacket. The standard .22 caliber is an example. A hollow point bullet has a hollow tip that makes it a highly lethal manstopper because it will pancake inside the body causing more damage to organs. Theese such bullets are outlawed by the Geneva Convention.
*Note FMJ bullets are not the same as armor peircing bullets.
Why on earth would any sane 13 yr old know this?
by BLOWYOURBRAINSOUT August 12, 2006
FMJ refers to ammunition, specifically for small arms (rifle & pistol) in which the bullet (typically with a lead core) is fully encased in a metal jacket (typically copper). Full Metal Jacket ammunition differs from a soft point or hollow point bullet whereby the tip is designed to fold or mushroom upon impact serving two main purposes: creating a larger cavity wound and preventing over-penetration of the target.
FMJ ammo on the other hand is less prone to deformity and tends to retain it's shape upon impact, penetrating the target and whatever obstacles may be in the way of that target.
The SAW gunner hit that skinny runnin' with an RPG and lit 'em up with a belt of fresh M82 5.56mm API FMJ tracer rounds.
by MossbergPump September 1, 2006
FMJ, short for "Fuck My Job", has been used to indicate a severe distaste for one's job, when one doesn't wish to express that distaste for all other aspects of his or her life, as then a more comprehensive and complete term FML would be used.
Person 1: "Why is it so hard to get to work on time?"
Person 2: "Because your job sucks."
Person 1: "FMJ."
by SlaveToTheWage August 26, 2009
Short for Fuck my jaw
I was at a Societies Plague show the other night and I was moshing. I got hit in the face and went to the bathroom to clean up the blood. My friend comes up to me after the bands set and says "Sorry I hit you in the jaw man!" FMJ!
by KBeebz April 27, 2009
Fuck My Job.

Expression used when bitching about ones job.
They made me clean the garbage cans at work today. FMJ
Ive been working since, 8 am, and I didnt even get a second to eat something or take even take a piss. FMJ
Zoe: "How's work going?"
Matt: "FMJ Zoe, FMJ. I am really over today, not one thing has gone to plan."
Zoe: "Oh. YOLO."
by Zoeeeee June 5, 2013
FMJ- Fingering, Making out, And jerking him off. All at the same time. He would be fingering the girl, She would be jerking him off, and you would be making out while all of this is going on. Its quite satisfying. It also makes an amazing sound. Two moans in harmony,
"this weekend my boyufriend and I tried FMJ, it was great,"
by Mj1232001 August 22, 2016