Munchkinism is similar to "Monty Haul" gaming; however it involves playing at incredible power levels purely for the sake of watching the terrain get blown away by player characters who are unstoppable. Munchkinism also involves "rules rape," wherein players milk every advantage out of the rules. Often a munchkin will carry a favorite character from game to game, usually with the maximum allowable ability scores, skill ratings, etc - and enough hardware/magic to destroy the planet four times over.
Source: wiktionary