CRAZY JUSTIN: PICHA ZA JUSTIN BIEBER AKIPIGA GUITAR UCHI

Our 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 a naked set a la Jenny from "Forrest Gump" ('memba that?).

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."

We're told G thought it was pretty funny -- and everyone in the home was cracking up -- but granny begged the pop star to get back in his room to put some damn clothes on, stat.

Thanksgiving -- they go balls out.