LMFAO! JAIME FOXX IN SOME SPEEDOS DANCING TO MICHAEL JACKSON!

Jamie Foxx does an AMAZING Michael Jackson impression ... in his underwear ... and TMZ has unearthed the unbelievable footage to prove it.

The video was shot at an after-party at Jamie's mansion in L.A. ... roughly ten years ago ... right around the time he was blowin' up for his role in "Ali."

Foxx -- sporting nothing but a pair of red briefs, white sneakers and a baseball hat -- gets downright funky in front of a group of friends while Prince's "I Wanna Be Your Lover" plays in the background.

Everyone had a blast that night ... and Foxx continued to dance his ass off even after the camera shut down.

That ni**a JaimeWhat kind of underware are those & did he give himself a wedgie @ the end

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