A cockimorto is when a triple kickflip is done correctly and your bike's frame swings around and launches one of your testicles through your scrotum and into Stewart Smith's Fire Hall Auction Spectacular. More generically, it's when your balls get hit by a part of your bike (which is metal, so it's gonna cause permanent genetic damage).

Shit, dude, your kids are gonna hate you for that cockimorto you just did!