1. a female with whom you have no interest in being engaged
2. a female acting in a slutty manner
3. a female lacking significant mental and physical attractiveness
'dude, check out that one in Old Navy visor...total dirtskirt'
'that girl is a dirtskirt for sure!'
Bro 1: Yo did you send out the invite for tonight
Bro 2: Yeah dude it’s gonna be a time
Bro 3: Did you invite the dirtythirty?
Bro 2: Ofc man, wouldn’t be a party without them
Any Californiamale who smokes weed and eats fast food, which normally ends up on a clean shirt. The person then becomes The Dirty Shirt, and will deny he is dirty regardless of how much crap he has on himself. To prove his point he will continue to wear the soiled shirt for days on end.
I saw Kurt in the garage smoking with Matt, and yes, Kurt is The Dirty Shirt tonight.