A punch that takes someone by surprise, a punch coming from out of the blue.

Easily confused with a punch defined as a "bitch move" a true sucker punch is quite a bit more complex than a simple unanounced attack.
It primarily involves a closed fist contacting the soft underbelly of a person (beneath the rib cage) at a high velocity, causing the ensuing force to press upward on the victim's diaphram, leading to a sudden expulsion of air from the victim's mouth and lungs. This opening blow leaves the victim open to various other attacks, often leading to what would be called "bitch moves" becuase of the defenseless nature of the victim.

When someone punches you and you didn't evan know you were fighting

This occurs when someone hits someone else from behind, usually when the person being hit doesn't know it until afterwards. Usually considered shady or a "bitch move".

A punch that takes your enemy by suprise, possibly knocking them out.

Comes from them being a sucker for not having their guard up

What Floyd Mayweather did to Ortiz on September 17 2011..

A sneaky punch from behind used for the element of surprise to incite the ire of prospective opponent. Usually it is just to shock the opponent, and not to cause any real damage.