argumentum ad grammaticam

Latin for "Argument at the grammar". To counter an opponent's argument, however the degree of validity, by simply correcting their grammar. A part of being a grammar Nazi or a pilkunnussija.

See also argumentum ad hominem ("argument at the person") or just ad hominem, where this term derives from.

"...And that's why your the worst."
"My the worst what?"
"And now you're resorting to argumentum ad grammaticam? That's lame."