To pwn someone with evidence.
Barney: (To Ted) Do you have some puritanical hang up on prostitution? Dude, it’s the world’s oldest profession.
Marshall: You really think that’s true?
Barney: Oh yea, I bet even Cro-Magnons used to give cave hookers, like, an extra fish for putting out.
Marshall: Ah ha, so the oldest profession would be fishermen. Kaboom! You’ve been lawyered!