Floyd Mayweather's Dad Cracked in Boxing Sesh, Knocked Down Hard

You barely EVER see a Mayweather get knocked down in a boxing ring -- but that's what happened when Floyd's 66-year-old dad strapped on the gloves this week ... and it was all caught on video.

Floyd Mayweather Sr. -- a former pro boxer who became one of the top trainers in the world -- was at his son's TMT boxing gym in Vegas this week, where he decided to spar with one of the younger guys.

It started off great ... with Floyd Sr. stickin' and movin' and throwing some pretty quick shots -- but, when the other guy started to fight back, it didn't go well for the old man.

Floyd took a few clean shots to the head -- sending him flying backward ... and ended with the elder Mayweather lying flat on his back with his feet up in the air.

Don't worry ... he's tough -- he'll be fine.