An Undercover Slut is a guy or girl who seems quite innocent and maybe even a bit prude but is actually a sexual deviant when given the chance to be. Being that the person is generally quiet and reserved about said deviancy, it's quite surprising when they reveal their true colors and you often find yourself very, VERY intrigued and wanting more. Just remember that they are, in fact, undercover, and have to uphold their mild-mannered alter-ego when exposed to the general public, and you must respect that. Also, don't assume that they've hooked up with tons of people- most Undercover Sluts like to reserve their sluttiness as to keep it meaningful and, obviously, undercover.
It's sort of like when the chick in the movie takes off her glasses and pulls her hair down and straddles you and you get the idea.

A term used to reference cannabis/marijuana/weed.

A girl that seems like the girl next door, just a normal girl. But, when she gets with guys/girls she goes all the way.

A girl who one would not assume gets with a lot of guys (whether because she is weird, nerdy, or just really nice) but in reality has hooked-up with half the school.

A girl/boy who appears very innocent but is a huge slut on booze/ behind closed doors

The girl that you wouldn't expect to be easy and hides her body count very well. Another word for Closeted Whore.