The tattoo one man gets of another man. The term originates from rapper Wale's tattoo of football player Chris Cooley. When someone gets a Cooley Tat they are gay.
Dude 1: "Ah man, I woke up last night and I had a tattoo."
Dude 2: "Oh shit, is that a tattoo of Mick Jagger? You got a Cooley Tat and you know what that means!"
Dude 1: "Shit, I guess I'm a raving homo."