Most of our grandmothers have seen our naked bums, by virtue of the fact that they changed our diapers. But most of us don't flash our rumps to our grannies at age 18. Unfortunately, that's just what Justin Bieber did.

Wait, what? WHY, BIEBS, WHY?

The Biebs engaged in a Naked Cowboy-like musical sesh for his grandmother on Canadian Thanksgiving in October 2012. He serenaded her in her Toronto home while wearing nothing more than a guitar covering the family jewels.

What was the point of such activity?

TMZ reports that The Biebs crashed out at his grandmother's crib during the holiday and overslept. He woke up after hearing family and friends doing their thing in the house and decided to ape the 'Forrest Gump' prank and play a naked set.

The singer reportedly sang some improvised lyrics, singing, "I loooove you grandmaaaa ... how are youuuu ... helloooo grandma."

Thankfully, since it was Canadian Thanksgiving, Grams had a sense of humor, as did the rest of the family. They still commanded him to get dressed.

The pics are NSFW. Even though he's not going full-frontal a la Lady Gaga, you can see his Bieber butt, which is naked as it was on March 1, 1994.

The pics remind us of that NYC Times Square tourist treasure known as the Naked Cowboy. Only he wears briefs and covers his bum.

Go ahead, Beliebers. Indulge your eyes on Bieber's buttocks.