Munchkin is how one should refer to someone who tries to win a role-playing game. It can be expressed, displayed, or seen in an aggressively competitive style of play. For example, getting the most enemies killed or the most loot taken, all without any thoughts about roleplaying, a storyline, fairness, logic and, the most importantly, other players' fun.
The term is used either as a negative label (for example, to an immature powergamer) or in a humorous or sarcastic context (like in "oh, what a munchkin you are, having 6d6 unarmed damage!").
Originally, the term was taken from The Wizard of Oz where Munchkins were short and ridiculous creatures. It was used by older players to address their childish comrades. However, before long it came to refer to anyone who engaged in a juvenile gaming style regardless of their height, age or experience. The term has been in use dating back as far as 1984.
Munchkins are infamous for various degrees of cheating, willfully misinterpreting rules that work against them while loudly proclaiming ones that work in their favor. As a matter of course they selectively obey the letter of rules while perverting the spirit blatantly. The worst munchkins will cheat shamelessly, ignoring inconvenient numerical modifiers and fouling dice throws till they get the result they want. During character creation, munchkins engage in vicious ''min-maxing, leading to exceptionally unrealistic or unusual characters that make no sense except in terms of raw power.
Munchkins are often accused of ''roll-playing, a pun on 'role' that notes how munchkins are often more concerned with the numbers and die rolls than with the roles that they play.
A more neutral use of the term does in reference to novice players, who, not know yet how to roleplay, typically obsess about the statistical "power" of their characters rather than developing their characters' personalities.
The term has gained popularity through the “Munchkin” card-game series. Originally published in 2001, the main game along with its many expansions and spin-offs offers players incentives to lower their friends’ levels, legally cheating and stealing, and eventually “winning” the game once said player reaches level 10. The rules of the game even state, "Any disputes in the rules should be settled by loud arguments with the owner of the game having the last word."
Such an attitude of valuing the improvement of the character's gear and stats over storytelling and immersion has also built its roots in computer games and MMORPGs.