Green’s Dictionary of Slang

(UK Und.) the victim, usu. a rural visitor to London, of a dice-player or a confidence trickster who uses counterfeit gold.

1552		G. Walker Detection of Vyle and Detestable Use of Dice Play 26: Be they young, be they old, that falleth into our laps, and be ignorant of our arte, we call them all by the name of a couzin; as men that we make as much of as if they were of our kin.
1591		Greene Notable Discovery of Coosnage 38: He that is taken, the Coosin.
1608		Dekker Belman of London (3rd) F1: The Barnard is the chiefe Player, for hee counterfeites many parts in one, and is now a drunken man, anon in another humour, and shifts himself into so many shapes, only to blind the Cozen.