Weakling was a derogatory term for a physically or mentally weak individual.
In 2267, Elias Sandoval assured James T. Kirk he'd find no weaklings (i.e. physically frail people) on Omicron Ceti III. (TOS: "This Side of Paradise")
In 2268, the insane Garth of Izar referred to weaklings as "inferior" and "decadent". He was tired of their prattle, felt those on peace missions constituted politicians and weaklings, and felt that the elite should grab power from them. (TOS: "Whom Gods Destroy")
In 2285, Kruge described David Marcus as a weakling Human. (Star Trek III: The Search for Spock)
In 2367, J'Dan believed that unlike the Federation, Romulans did not turn Klingons into weaklings like themselves. (TNG: "The Drumhead")