A rules librarian is an individual who has scoured the rule-books of a game to find every advantage and every loophole that they can to "empower" their character.
Most often found in Role-playing games such as Dungeons & Dragons, but variants can also be found online playing MMORPG.
These people suck the fun out of any game due to their obstinate refusal to actually get into the spirit of the game and play a ‘character’.
Their main motivation is to ‘win’ and if they don’t, or other players enforce the spirit of the game rather than the rule they will generally sulk or threaten to leave.
On dealing with a Rules Librarian
1. don’t play with them
2. if you already are – kill their characters off until they learn, or stop playing
Rules Librarian rolls dice...
"Ok, so you swing at the 50 foot tall dragon and strike a nasty blow to it's leg"
"Actually, because I rolled over 95% of my base strength score; coupled with my level and weapon specialization, and my Backstab bonus as a thief, I get an automatic triple critical, so seeing as this is a Blue dragon, with a maximum of 200 Hit points, I would actually kill it"
"It's a dragon, you can't kill it with a dagger"
"It's in the rule book; Page 60, 193, and 246"
(other players groan)