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Someone who doesn't appear to be slutty at first glance but in fact is a huge slut upon further investigation.
Observer #1 "Hey man, check out that girl over by the bar. She looks like the only classy girl here."

Observer #2 "Yeah, normally I would agree. But I know that girl, and she's a Sleeper Slut."
by DaiquiriDude April 12, 2012