To "correct" someone in a very smug, self-righteous way while providing wrong information yourself. Originated by Fark user of the same name with such gems as:
rotsky: Uhh. First, check your math. 150 gallons (of water) = 15,000 lbs.
Bob: I'm going their tomorrow.
John: You jackass. It's "I'm going they're tomorrow."
Rick: John, you magificent bastard. You rotsky every person you talk to, don't you?