On Fridays you wear a shirt which you bought at a thrift store like Value Village and wear it every Friday without washing it until it is unwearable in which case you buy another.
Justin: That shirt looks terrible it must be Dirt Shirt Friday.
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!'