An attack on an opponent's grammar to completely discredit his/her argument.
See Ad hominem.
Faiz: I spent a good half-hour typing out an argument for nothing, because someone used Ad Graminum and pointed out my "your" that should have been "you're".
Julian: You deserved that.
Brian: The Lakers are fundamentally a much better team, they're offense is-
Julian: HA! "THEIR"!
Brian: Fuck you.