A duck off is a
girl/boy who is used for forms of sex in private but is rejected in public.
Some
may be considered 'sluts' while others are average
girls. Only about half of them actually know that they are duck offs.
Sam:She's beautiful, are they dating?
Jordan: No way, she's just his duck off.
Sam: Well I can't say much, I've been the duck off before.
Jordan: Me too
girl.