Leeroy Jenkins

Leeroy Jenkins is a lone-word/meme that originated on /v/, but which has been readily absorbed into the /tg/ lexicon. A Leeroy Jenkins is an individual who constantly ruins battle plans by charging headfirst into battle, no matter the odds, usually because they're too stupid or impatient to wait or to try and keep to any sort of strategy. A particularly annoying variant will actually pick fights needlessly just for the sake of having someone to fight, usually because the player finds planning, talking, etc to be "boring". Leeroy Jenkins are, ironically, rarely munchkins - or at least not very good ones, since your typical munchkin wants to "win" and understands that blindly attacking every foe in sight isn't likely to succeed.

The 1e Dungeons & Dragons Cavalier-Paladin earned its particular amount of scorn because it took the Paladin's "crunch-mandated fluff" of Lawful Stupidity and enforced this very behavior in the exact same way. To whit:

As a result of the code and the desire for battle, cavaliers cannot be controlled in battle situations. They will charge any enemy in sight, with this order of preference: 1. Powerful monsters (dragons, demons, giants) serving enemy leaders. 2. Enemy leaders. 3. Opponent cavaliers of great renown, and enemy flags and standards. 4. Opponent cavalry of noble or elite status. 5. Other opponent cavalry. 6. Opponent elite footmen. 7. Opponent camp and headquarters. 8. Opponent melee troops. 9. Levies or peasants.

So, you have a class that is not only expected to act like this, but which gets game penalties if it doesn't. Not a lot of people approved, obviously.

The Leeroy Jenkins meme began with a World of Warcraft fan-video that was intended to mock overly complicated plans and "overthinkers". Instead, the video's protagonist has been adopted as the villain on /v/ and /tg/ simply because that particular flavor of blindly aggressive player is well known in both circles as one of the most common and annoying branches of That Guy's family tree.