an affirmation that something is cool, amazing, hardcore or unbelievable...kinda like asking "are you shittin me?" or saying "wow, holy shit dude"or "hell yeah" depending on if it is used as a question or a statement
1. bob:hey the red sox won the world series
billy: fuck around??
Bob: i shit you not dude
2. bob:yo dat party was off da hook yo
Billy: fuck around!! fo' real yo