Grammar Nazi

A person who is willing to completely kill the momentum of intelligent discourse to moan about the location of a preposition or comma in a sentence. They usually aren't as smart as they think they are, so it's their way of interjecting something they feel has intellectual value into the conversation.

We were having a really great talk about the theory of existentialism until John the fucking grammar nazi completely ignored your compelling argument just to point out that you shouldn't use a preposition at the end of a sentence.