What is a “wedgie”?

I lost a bet with a friend, and he said next time I’m over at his house (which is in a few hours) I have to let him give me a huge wedgie. We’re close friends and I’m sure it’s some sort of prank or something😭, I just don’t know what to expect and I feel dumb for not knowing what it is lol. I thought a wedgie was when your pants got stuck up there while you’re walking.