Originally means "fixed that for you." It also works as a double meaning for "fuck this fuck you." It's only used in that derivative in closed circles to confuse those that aren't in "the know" and is usually used in cases when two parties argue over something innately idiotic that has no logical outcome.
User 1: This game is awesome!
User 2: "This game sucks!" ftfy
User 3: ftfy
User 1 and 2: lol wat