A Punch-Out is a slang term in professional boxing referring to a boxer who boasts and taunts his opponent but can not hold his own in the ring in actual boxing competition. Informally defined, it could be best described as being able to "talk the talk" but not back it up in the ring.
The term is famously used in a quote the film Rocky II by Burt Young playing the character Paulie, who instructs Rocky to "Blow this punch-out!" in response to Apollo Creed's taunting at a press conference.
The term was also used as the name for a popular arcade game titled Punch-Out!!, and then later a spinoff of the game for the Nintendo Entertainment System titled Mike Tyson's Punch-Out!! The name reflects the tongue in cheek, comical style of boxing seen in the respective games.