Mark

From WIN-ipedia - information and strategies about winning

"Mark": slang term for the victim of a confidence game/trick.

Historically, the victim would literally be marked by one of the operators of the con. Usually this involved a member of the 'gang' spotting a potential vicitm, noticing he ahd ready cash on him or discovering in some other way that he was gullible, the operator would accidentally brush against the victim's arm or shoulder. The purpose of this, however, was to leave a dusty chalk mark, or (in some cases) a sticker on the 'mark's shoulder or back. This way, other operators could easily recognise that the man was good potential for a con.