OLD GUY FIGHT and it was all over the CFL!

The wrinkled, arthritic fists were flying at an event for the Canadian Football League this weekend -- when two 73-year-old former players ATTACKED each other ... and it was all caught on tape.

Long story short -- former NFL quarterback Joe Kapp (who also played in the CFL) came face to face with former CFL enemy Angelo Mosca during a luncheon for the Grey Cup (the Canadian version of the Super Bowl).

Kapp shoved a flower in Angelo's face ... Angelo swung his cane ... and then Kapp fired off a NASTY right cross. Boom. Old man goes down.

According to reports, these two have hated each other since 1963 ... when Mosca threw a cheap shot at one of Kapp's teammates.

Revenge is a dish best served cold .... really cold ... with a side of prunes.