Basicly any sexual act. Normally used when the sexual act isn't clear.
I heard him and Kate slapping the nasty last night

Damn, you're sweaty, have you been slapping the nasty?

He likes to slap the nasty with cherry-flavored lubricant.
by Baby Gloomi April 6, 2006
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When something is so nasty you just want to slap it. Most commonly used as an adjective.
Boy: "I want to eat your FUP for dinner"
Girl: "You're so slap nasty"
by Tonya62 July 25, 2006
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a insult to someone meaning that they are a discusting, vd filled, whore
" Man I brought home a SLAP NASTY HOE last night and now my ball itch
by the big jo- to the nizzle July 23, 2003
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