An exclamation of when the penis becomes erect and is pulled from the inside of one's underwear for viewing. It is a term that is generally used by the driver in the front seat suggesting that his friend is ready for horseplay with his bitch in the backseat.
Mike: Hey Tony, so that was some concert, huh? Boy, that Blue Oyster Cult can rock, man that cow bell was..was..what the? (twists head to the backseat) COCK OUT! COCK OUT!