The basic name for the Half-Life 2 modification Garry's Mod. Garry's Mod is a construction mod and MingeBags typically ruin the game. MingeBags are often referred as assholes, 12 year olds, and n00bs. Most often they will join your server or game and generally fuck up your constructions.
Extreme caution is recommended when allowing these users to join your game. "OMFG THAT MINGEBAG JUST DELETED MY CATAPULT!!!!"
"Goddamnit, someone kick that fucking MingeBag! He keeps killing everyone!"
"KICK THAT FUCKING MINGEBAG BEFORE I KILL HIS FAMILY!!!!111one
Taken from: http://www.urbandictionary.com/define.php?term=Mingebag