I don't know where this happened, but there's a new video on YouTube of some guy who found out his underage daughter had been having sex with her ADULT boyfriend.

But instead of calling the cops, he went to the guy's house, made him lie face-down on the ground and SPANKED HIM.

In the video, the dad's actually calm the whole time, and tells the guy that if he doesn't want to get spanked, his other option is to get a BEATDOWN.

The boyfriend doesn't look like much of a fighter, so he opts for the spanking. And the dad gets about 20 solid whacks in...then the guy starts crying.

The best part is at 1:52 when the guy cries and says, "I'm not gonna be able to sit, dude!"

WARNING: This video includes the F-word, S-word, A-word, and backside nudity.