A person who takes it upon themselves to correct the grammar of not only their close friends but also causal acquaintances, often at the expense of disregarding surrounding circumstances.
"Dude, where was you last night?!? I saw your girl at the club getting all up on some other dude. She totally left with him and his buddy, yelling something about a threesome."
"Don't you mean, 'where were you last night?'?"
"Dude, who are you? The Word Police??"