Sucker punch
A sucker punch (American English), also known as a cheap shot, coward punch, king hit, dog shot or one-punch attack (Australian and New Zealand English), is a punch made without warning or while the recipient is distracted, allowing no time for preparation or defense on the part of the recipient. The term is generally used in situations where the way in which the punch has been delivered is considered unfair or unethical, and is done using deception or distraction, hence the term 'sucker' used to refer to the victim.
In boxing, a sucker punch—as is done when 'hitting on the break', for example—is illegal. For example, when James Butler knocked Richard Grant unconscious after losing a fight to him on points, his license was suspended and he served four months in prison. It is often thrown from behind—such as in the 'knockout game'—although striking from behind is not a prerequisite for a sucker punch.