Sucker
Sucker, dupe, chump, turkey, or  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. ()

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  posing as Ardra's deception. ()

In 2375, in giving his order to Tony Cicci to shoot Vic Fontaine, Frankie Eyes said "Bury him,", then addressed Fontaine, "So long, sucker." ()

Societal classifications
Insults