Sucker was a term used to describe someone who either had a weakness for certain types of people, fell for con games easily, or was just plain out of luck.
A famous saying attributed to P.T. Barnum was, "There is a sucker born every minute." In 2367, Jean-Luc Picard quoted Barnum to Data when considering the impostor posing as Ardra's deception. (TNG: "Devil's Due")