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