1. Interjection used to express disbelief at an understatement. Obviously; no shit.
2. Statement used to express general agreement with a friend. Fuckin' A'; for real; I heard that.
(Chet can be dropped in any usage)
1. Bill: Your leg is on fire!
Tim: No guff, Chet!
2. Carl: We really stepped in it this time.
Joe: No guff.