Marshawn Lynch -- Never Coming Back To NFL ... Says Rap Star Cousin (VIDEO)

Pro football is just "not fun" for Marshawn Lynch anymore ... so says his famous rap star cousin who's 100% positive Beast Mode is NOT returning to the NFL.

Marshawn's cuz Mistah F.A.B. sat down with with "The Breakfast Club" -- when Charlamagne Tha God asked about rumors that Lynch is thinking about coming back to the league.

"He's done. It's not fun to him no more," F.A.B. said.

Long story short ... F.A.B. says Marshawn is super wealthy (saved his NFL cash like a boss) and has been spending his time building schools and affordable housing across the world.

In fact, F.A.B. says Lynch wants to mentor young players so they can get to "mogul status" too. '