Before Odell Beckham hit the gym with Dez Bryant -- he hit the stage with Shawn Mendes for a rap duel on "Drop the Mic" ... and it's pretty great.
The NY Giants star went after Shawn's whiteness ... a lot ... with some lines that included:
"Your skin is so pale, let's call it a hunch ... you're so white, the only meal you eat is brunch."
Also ... "No matter what you say my ego won't burst because whatever you've done, John Mayer did it first.