Ballthumb
The situation arises when a male uses the washroom, and when exposing himself to urinate, contacts his balls with his thumb. This results in a recognizable nutsack-like odor on your thumb. Preventable by handwashing and using the lil hole in the front of your boxers.
I was on a date with Steve last night, and everything was fine until halfway through dinner; he came back from the washroom and all I could smell was his ballthumb