Sucker Punch Meaning

(idiomatic) A disabling punch targeting a place which is not normally acceptable in a "fair fight", such as on the back of the head.

Example: 2011 January 8, Chris Bevan, “Arsenal 1 - 1 Leeds”[1], BBC:
  The Championship highflyers almost got their reward for a resilient performance on their first visit to the Emirates, surviving a flurry of first-half Arsenal chances before hitting back with a classic sucker punch.