Mystery Nut

When you shoot jizz during a particularly hot masturbation session, only you shoot wild. SO, after washing, you come back to the scene of the crime to clean up your cummy mess, but there is literally nothing there. You just shot a mystery nut.

"I shot this load over my shoulder, and I know it landed somewhere behind me, but I can't find it! It's gone. I've felt everywhere."
"Bro, you just shot a mystery nut."