Floyd Mayweather Threatens To Slap Conor McGregor

Floyd Mayweather and Conor McGregor's war of words just got realer than real ... with Mayweather vowing to "slap the s***" out of the UFC star the next time he sees him.

Mayweather made the threat while doing an Instagram live chat on a boat (in full Hawaiian shirt mode) ... saying he's tired of Conor mentioning his name and he's now willing to take violent action.

This comes just days after TMZ Sports got McGregor out in L.A. dissing Mayweather yet again ... saying that it's himself -- not Floyd -- who's the biggest combat sports star in all the world.

Floyd was smiling when he made the statement ... but we know The Notorious one doesn't take any threats lightly ... and has had near-brawls with other UFC stars based on Twitter beefs in the past.

It's starting to get good, folks.