Just because Gene Hackman retired from acting a few years ago doesn't mean he can't summon some of his signature grit when needed.

TMZ reports the 82-year-old Oscar winner had just left a New Mexico restaurant with his wife yesterday when they were approached and threatened by a homeless man named Bruce Becker.

The rest sounds like a scene from one of the actor's tough guy films.

We're told the incident escalated big time when Becker called Hackman's wife a "c**t" -- and then Hackman dished out some street justice ... slapping Becker across the face.

Becker then called the cops to report the incident.

We're told cops interviewed Hackman and his wife, determined the slap was in self-defense ... and, for now, no charges have been filed.