Used as a synonym in the context of an expression equal to "not much" or "very little". Only really applicable when asked a question such as "what's up?" or "what are you doing". Based on the stereotype that the Japanese have small penises, and played on the modern response "jack shit".
Mike: Hey man what's up?
Shawn: Jap dick.
Billy: Yo, what are you doing?
John: Jap dick.
Billy: That sounds pretty gay.
John: No, it is an expression. It means not much or very little.
Billy: Oh, I get it! Like a Japanese dick! HAHA!