1. A noun, purpura is male genitalia that is all testicles and no penis or a very small penis overshadowed by deformed balls.
2. A deformed penis.
The word originated in New York amongst Italian kids who would make fun of a kid with a deformed penis and grew to describe some with "all balls and no shaft"
"I heard Carol left John because of his purpura."
"Scott's purpura keeps him from getting any"