40 Glocc -- Poppin' Mitts With Mayweather's Uncle ... Training For Celeb Fight Night

Mickey and Rocky eat your hearts out ... 'cause rapper 40 Glocc and Floyd Mayweather's uncle, Jeff, have formed a new pugilistic palship ... and TMZ Sports has the video of their first training sesh.

40 Glocc is stepping into the ring with 50 Cent's associate, Jack Thriller, on July 27th for the first ever "Hip Hop Fight Night" celebrity boxing event in Atlantic City ... and it seems like he's taking it seriously.

How serious? So serious that he's enlisted former IBO Welterweight champion Jeff Mayweather -- Floyd's Uncle -- to train him for his celebrity scrap.

Check out the clip -- Jeff doesn't seem at all impressed with 40 Glocc's hand game ... and has to use a little tough love to the big bad 4-0 to throw some real shots.

At least 40 got the chance to throw some shots this time ... his last foray into fisticuffs didn't go as well.