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.

In 1986, Gillian Taylor explained to James T. Kirk that she'd gone out to dinner with him because she was a sucker for hard-luck cases. (Star Trek IV: The Voyage Home)

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")

In 2375, Frankie Eyes said, "So long, sucker". (DS9: "Badda-Bing, Badda-Bang")