Having anal sex for the first time. Also could refer to losing/popping the "ass cherry". Normally used by a guy when talking about having sex with an anal virgin
Other versions include:
Popped the trunk, Popping her trunk, Popping his trunk, Popped my trunk
Guy 1-"Hey dude, you gotten your girl to give up her cherry yet?"
Guy 2-"No, but tonight I'm going to be Popping the trunk."