The phrase used to describe a very promiscuous guy, particularly with one who tends to get with women who aren't exactly "clean". The "D" stands for "Dick", giving hint that he may, or at least believed to be, carrying a venereal/sexually transmitted disease from all his sexual interaction with various women.
Michelle: I like John and all, but he does have a reputation of being a playa
Stephanie: Yea, I heard he has a Dirty D from all those girls he'd be fuckin'.....so watch out
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