kick in the pants, a

1.  Also, a kick in the teeth. A humiliating setback or rebuff. For example, That rejection was a real kick in the pants, or That review was a kick in the teeth. A third, vulgar variant of these colloquial terms is a kick in the ass. Versions of this last expression-- kick in the breech, kick in the behind--have been used since the early 1800s.
2.  A cause of enjoyment, as in That show was a real kick in the pants. This meaning is virtually the opposite of def. 1 and can be differentiated from it only by the context. [1960s]