Said of pants or shorts that are so tight you can clearly see the outline of someone's junk. Junktight is most often used to describe male clothing, and only when the speaker doesn't find the sight to be intrinsically gross.
Aaron Behrens, the singer for Ghostland Observatory, is known for madly gyrating around the stage in junktight jeans.
The Tron Guy's outfit is not junktight, it's just fucking gross. Lose some weight, man, no one wants to see that.