A girl under the age of 18. (actually anyone under 18); Minor in the U.S. When informing another unknowing person that the girl they are peeping is under age you tell them, "she's tu yong" aka too young, with a Chinese accent to make this sometimes awkard moment funny yet surreal. Having sexual relations with a young girl is illegal and get you thrown in jail, you caught the Chinese Disease.
Pancho: Whatcha lookin at dude?
Theodore: Dat chick over there, man she's fine.
Pancho: Hey, she's got that Chinese Disease, better stay away.
Theodore: What is that, is she from China?
Pancho: No, homeboy she's tu yong.