"The Hangover" star was filming an episode of his quirky Internet series "Between Two Ferns" in 2014, featuring the "Baby" hitmaker, when he decided to shower in the singer's trailer - and he made off with a weird keepsake.
"I had to change clothes... (so) I got into his trailer... I didn't have any underwear, so I see a pair of underwear in the corner," the funnyman says on a new episode of David Letterman's Netflix show "My Next Guest Needs No Introduction with David Letterman". "I needed a pair of underwear... I put 'em on. Then I started thinking about it. (On) a lot of levels it's wrong."
Galifianakis continued to shock Letterman when he explained he kept the intimate item and planned to give the underwear to his nieces, who were huge Bieber fans.
"I took the underwear," he adds. "Years pass, and I had taken the underwear with me... because I have nieces."
But the gifting experience wasn't what he expected when he eventually tried to hand the pants to his sister's daughters.
"So we walk up to my bedroom, my sister and my niece..., and I'm like, 'Hey, I've been saving these for you'," he recalls. "And the look on my niece's face... She did not accept it... She was not into it at all."