A character in Dungeons & Dragons games with a "Lawful Good" alignment, played poorly and to the detriment of the game, usually resulting in the deaths of the character and his allies.
(See also: Chaotic Stupid)
Lawful Stupid Player: My Paladin detects evil on the wizard, and attacks!
Other Players: But he's got an ogre and ten orcs...
Lawful Stupid Player: I smite the Wizard.
-The party is subsequently massacred.-