Justin Bieber NAKED — It’s My D**k In a Guitar! (Photos)

Exclusive 0811_justin_biever_article_wm_naked_tmz_2Justin Bieber is wise enough to know when a gift needs givin” … but he didn”t have a box — so he covered his junk with a guitar and jammed out a naked serenade for one lucky lady fan — HIS GRANDMA!

our sources has obtained photos of a completely nude Biebs  strumming away on his big wooden instrument … at his grandmother”s home in Toronto during Canadian Thanksgiving back in October 2012.

And why, you ask, did Bieber go ass naked for g-ma

0811_justin_biever_article_wm_naked_tmz_1Our 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.

Canadian Thanksgiving — they go balls out up there.