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.