Q.E.D.
If somebody declares there's too much cussing on the board, and someone else replies, "Like fuck there is!" the first person might reply with "Q.E.D."
It's kind of shorthand for "See? Right there it is in front of everybody. What did I tell you?"
It's not so much that you've made your point under this circumstance, it's that your opponent just made it for you.