crusty gusset
The gusset is the part in your selected underwear, between your genitalia and your rectumnal cavity.

Crusty is crusty, so when you have a crusty gusset a layer of thick flakey substance builds up. Usually made of specimens such as arse cherries (shit on the end of a pube) and male sexual juices. Even if u are of the inferior gender (female) in which case stop reading this and get back in the kitchen. You binty slag.
Cor her gusset were dead crusty mate. She had a dead janky ass crusty gusset.