The slang term "mark" applies to a person who is perfectly set to be victimized. It is used mostly by
carnival workers and
magicians .
For magicians, a "mark" is whomever they are planning to pull their
trick on. The term comes from a little known practice formerly employed by shady carnival operators (and possibly still employed in some places today). As a
patron took out their wallet to buy tickets prior to entering the
carnival, someone, usually a manager of sorts, would watch carefully to see if they had a lot of money. If they saw the wallet bulging with
greenbacks, they would walk over and pat the person on the back as they entered. Concealed in their hand would be a piece of
chalk. By patting them on the back with the chalk, the carnival worker gave them a "mark" which told the
pickpockets who to strike.