A sexually alluring, seductive, and flirtatious adolescent girl younger than the legal age of consent to sexual intercourse (in the US, 16-18). Such girls are known as "jail-bait" because if a man (legally, of any age, but colloquially considered to be an older man - relative to the age of the girl) has sexual relations with her he could be arrested for statutory rape (sex with a minor).
Dude, that hot red-headed Lolita in the booty-shorts and halter top has been coming on to me all night and I really wanna get with her. She smiled and winked at me and said she was legal, but she can't be a day over 16. She's totally jail-bait, and I don't want to end up on the National Sex Offender Registry; I'll never get laid again.