kick in the pants, a

1. 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]

2. 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.