Unsanitary or unsighly genitalia, usually resulting from the contraction of some sort of venereal disease. Yuck junk characteristics may include foul odors, discoloration, bumps, lesions, etc.
"yeah, she's cute and all, but she has got some SERIOUS yuck-junk!! As crudded up as that girl is, you would be better off fucking a monkey from Sub-Saharan Africa........ or a Haitian refugee. You should wash your dick for even thinking about it!!"