someone who secretly is badass at something but doesnt let anyone know

The term 'sleepers' is simply another word for 'pajamas'. Personally I think sleepers is a funnier phrase therefore I use it constantly.

Usually used when describing a movie, especially one that seems boring, mundane, or overdone--but it ends up becoming a huge hit. From what I understand, the original "Dirty Dancing" was a sleeper hit.

railway car with sleeping berths and/or bedrooms ("roomettes")

It's boring. Puts you to sleep.
The term most likely derives from the twitch emote residentsleeper

A girl who is usually unattractive and after a few years "grows in" to herself and seems attractive.

A girl who is plain and average looking that you would not give a second look at but if you sleep with her, you find out that she has an incredible body and is exceptional in bed at pleasing a man.