When a girl wears an item of clothing sold as a shirt as a dress (ie. without pants). Shirts for skirts is often coupled with ass hanging out, or constant tugging at the bottom of the shirt to prevent this occurance.
Jack: I can see that chick's chouger
Jack and Jill (heckling): SHIRTS FOR SKIRTS!
Jill: That girl has the same shirt as me...
Jack: Yeah but she's not wearing pants! shirts for skirts!