"jap dat" has its origins rooted in the australian city of Melbourne. jap dat is a casual tournament of american football between students from two schools. it is arranged by students and is often played on the weekends, there are also jap dat fights were two students put boxing gloves and spa for three rounds.
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!
The rare occurrence where only the skin of middle part of your jap's eye sticks together. When you pee, two streams come out creating the illusion of a double jap's eye.