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1.
The act of assuming someone is a slut, when they're actually not.

Also used for assuming that someone is not a slut, but they actually are.
You find out the "innocent" girl next door has screwed over 50 guys:
"My slutsumption was wrong about you, you really are a slut"

You see a girl at a strip joint only to find out she's a virgin:
"My slutsumption was wrong about you, you're not a slut at all!"
by Dale The Awesome October 02, 2011