Floyd Mayweather -- Scoffs at Conor McGregor ... That Guy's a Joke

Floyd Mayweather doesn't give a single DAMN about Conor McGregor -- telling TMZ Sports the UFC fighter is a joke ... who's using his name to try to get some cheap publicity.

Earlier this week, McGregor said he'd DESTROY Mayweather in a fight in less than 30 seconds -- because Floyd wouldn't be able to handle an MMA-style attack.

But sources close to Mayweather tell us ... Floyd "scoffed" when he saw the quotes and said, "I don't take that dude seriously. He's just trying to get himself some publicity."