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1.
A person who lacks intelligence and common sense and is outrageously promiscuous.
"How does that girl not realize that that guy doesn't want to hook up with her...and that three of her friends have already slept with him?! What a dumb dirty."

"I can't even stand that bitch. She's such a dumb dirty."
by For You MJ August 31, 2009
 
2.
The act of doing a promiscuous activity, either alone or with one other persons, infront of an audience or crowd of spectators. Doing the dumb dirty will usually result in extreme embarassment and shame.
vinny: you abdu, how you gonna do that dumb dirty in front of all these kids at the part?
abdu: i'm am so ashamed, why am i dumb dirtying??
vinny: bruh nerd
by pharohdick May 26, 2014
 
3.
nounDum-Derty - A person who is dumb and dirty at the same time. It makes saying it faster, condensing it into one word. Instead of saying, "wow that was dumb and dirty at the same time" you can now say, "DUMBDIRTY"
Chad: Your so uglyyyy
Carmen: wow just shut up
Matt, Pete,Jan: That was just DUMBDIRTY.
Carmen: Ohh my god....
by Kanapp February 01, 2011