50 Cent -- Trolls Floyd Mayweather ... Nelly Stole Your Chick!!

50 Cent is talkin' some SERIOUS SMACK to Floyd Mayweather ... harassing the boxer over the fact Nelly is dating Floyd's ex-GF ... and claiming the rapper "really took your lady."

For the record, 50 and Floyd used to be tight -- super tight -- but they had a bad falling out ... and now the rapper is trashing his former friend all over social media.

It started when 50 posted a pic of Floyd's ex-GF Shantel Jackson -- writing, "CHAMP I ain't gonna lie this motherf**ker bad. You need to start operation GET THE B*TCH BACK. Right now!!!"

Then he fired another shot -- posting a pic of Miss Jackson with her boyfriend Nelly ... and added the caption, "Nelly put this up @floydmayweather DAMN da n**ga really took your lady Champ."

So far, Mayweather has yet to respond -- but you gotta imagine, he ain't happy.