A penis that smells 'bad'. The smell can resemble many off-putting things, like cheese, fish, sweaty clothing, or anything else that smells rotten. It is considered unattractive and gross, especially when one is near a stinky willy, for example, when in sexual intercourse. A stinky willy usually occurs when one does not wash ones self for many days, or after eating a stinky meal like a curry, or doing large amounts of exercise without washing. A stinky willy can be remedied by having a good wash, and spraying deodorant in the groin area. The equivalent for a woman is a fishy pussy.
I fancied this guy so much, but after smelling his stinky willy, I dumped him.