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1.
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.
by jschoo February 05, 2012