we do not use this term for girls, we use it for boys. we use this term when boy's are wearing pants that are so tight on the legs and the crotch that they look like girl's pants. or maybe the boy even bought these pants from the girl's section in a store, or borrowed them from their sister or girlfriend. boys wearing girlpants has become a fad in rock bands and in skateboarding.
"Hey look Joanie! Those pants are so tight on him, they must be girlpants!"