sleeper car /slēpər kär/ (noun): A chick that looks like a prude and seems shy, but as soon as she has a few drinks or you get her alone, she's up on in!
Dude 1: "Man, that chick I met at the library last night was a free-eak!"
Dude 2: "The chick in the frumpy brown sweater?"
Dude 1: "Total sleeper car, bro."
Dude 2: "Nice!"