sucker punch 
informal
noun
An unexpected punch or blow.Pushing ensued and then O'Neal made like the other O'Neal and threw a sucker punch....
- In training camp, he crushed the eyesocket of a teammate with a sucker punch and by the end of the season he was ensnared in the THG drug scandal and contemplating retirement after a series of concussions.
- When you're used to being patted on the back all the time, a devastating counterargument feels like a sucker punch.
verb
(sucker-punch) [with object] Hit (someone) with an unexpected punch or blow: his father sucker-punched him and knocked him out...
- He was sucker-punched, a bottle was broken over his right eye, a knife was plunged through the back of his leather jacket six inches deep into his body and he received 10 other stab wounds.
- I don't mean to sucker-punch every coach's son - just the ones who deserve it.
- On one assignment, I decided to sucker-punch my favorite teacher.