A girl who is always available to have sex. From the Gutta Boys song "Now or Laters": "I want them Now or Laters, them Now or Laters, that's them girls that'll give it to you now or later."
"Don't trip on that girl, she's a now or later, she'll be ready when you are."
is when the boy gets the girl in the mood and they both hot and ready but when the boy inserts his penis and strokes for a couple of SECONDS his penis goes back on soft and the girl gets mad and walk away
man that was shit i could have fucked somebody that can stick and stay with your now or later dick