Justin Bieber Plays Guitar For His Granny While Naked...

sources tell us JB had been staying at his granny''s house during the holiday ... and slept in a little too late on Thanksgiving morning.

Bieber finally woke up after hearing family and friends stirring around the house -- so he thought he'd prank 'em ... by grabbing his guitar and playing naked.

We're told birthday-suited Bieber went right up to his grandma and started belting out some impromptu lyrics ... like, "I Loooove you grandmaaaa ... how are youuuu ... helloooo grandma."

If you ask me, I think this kid has finally lost it.