sucker punch

Short definition from Wiktionary

Noun

1. 1
   An unexpected punch or similar blow.
2. 2
   A punch or jab on an opponent who is off guard or not active.
3. 3
   A disabling punch targeting a place which is not normally acceptable in a "fair fight", such as on the back of the head.

Verb

1. 1
   To deliver an unexpected blow.
2. 2
   Alternative form of sucker punch.