Fat Joe says he truly believes 50 Cent and Floyd Mayweather are "brothers" and need to end their feud -- but he's not the gonna be the mediator.
Joe is friends with both guys -- and says it's crazy to watch them bash each other so hard on social media.
The latest in the feud ... Floyd says 50 Cent is a "herpes infested rat." 50 says Floyd is a fake-rich "sucker" who beats women.
50 also made some dark accusations about Mayweather's alleged role in the murder-suicide of one of his friends.