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."

when you're hungover as shit after a spending a night drunk as a skunk

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