Top Definition
(Adj.) A personal description related to the whorish or slutty acts willing to be performed for one's partner only on their birthday, birthday-only acts being more extreme than everyday acts.
1. Your mom is beyond nasty - she's birthday dirty. She frightened me a little last night...
2. My ex was a little drunk last night, so I figured why not get birthday dirty for a little while...
by The Basement Office May 07, 2010
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