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1.
When you sleep with people in a stupid manner or when you act incredibly dumb about your sex life.
Guy 1: Man, last night I was so drunk. I thought that chick I slept with was Larry's sister. But guess what?

Guy 2: What? It was actually Larry?

Guy 1: No it was my dog.

Guy 2: That was pretty dumbiscuous.
by Andrewlicious March 06, 2012