Why Did Bieber Go Naked For Grand Ma ?

Justin Bieber is wise enough to know when a gift needs giving' ... 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!

TMZ 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?
tmz.com