Either the bravest SOB in World of Warcraft history, or the stupidest; depending on your point of view. Rushed into an enemy stronghold alone, roaring his name; forcing the rest of his guild to follow and turning their chances of victory from 32.33(repeating)% to zero. At least he has chicken.

To "pull a Leeroy Jenkins" is to rush into a dangerous situation alone without following strategy, forcing your allies to follow and leading to the inevitable, definitive, graphic demise of your team. In essence, your entire team got wombo comboed because of you. If you don't try to laugh this off by saying, "At least I have chicken", or at the very least acknowledge your foolishness, you not only fail at the game, you fail at life.
Note: NEVER try to pull a Leeroy Jenkins if you're an actual member of the military. Chances are no one will make it out alive. And you won't even have chicken to comfort you.

one who joins an xbox live team based game for the sole purpose of screwing it up. full of annoying, repetitive conversation or loud c(rap)music. usually a friend of someone on the other team sent to give info to them or sabatoge by team killing, taking all the weapons on a map, stealing vehicles and ditches them for the enemy, destroying team assets, texting or voice chatting with the other team giving them teammates positions or plans, intentionally dying for the other team to get the win. a childish gamer. anyway you slice it acts like a leeroy jenkins.

to spontaniously run head first into FAIL.
you have only acomplished a Leeroy Jenkins when you epically fail at something almost instantely and spectacularly and then state 'at least i have chicken'

Baddest man in the whole damn town. Badder than old King Kong, and meaner than a junkyard dog.

Is the term to NOT do things but at least be in possession of chicken

1. A character from the popular MMORPG World of Warcraft (WoW) played by Ben Schultz.
2. A card in the World of Warcraft Trading Card Game with the special ability of exhausting all other creatures. If you call out "Leeeeroooy Jenkins!" the card can attack instantly, but has low health and will generally die fast. Similar to the character in the game.
3. An incredibly stupid act that generally results in suffering for others.
4. A catchphrase used by players of various different MMORPGs when they are about to commit suicide, i.e. Its Leeeroooy Jenkins time!

To spontaneously throw caution out the window and run into battle yelling "LEEROOOOY JENKIIINS!"