A sling was a folded piece of fabric that supported an injured body part, usually an arm.
Trip Tucker's left arm was covered with a sling following the injuries he sustained on Mars. (ENT: "Terra Prime")
In 2268, Leonard McCoy complained about wearing his dress uniform, saying he felt as though his neck was in a sling. (TOS: "Journey to Babel")
In 2294, Montgomery Scott made a makeshift sling for himself after injuring his left arm in an accident on the USS Jenolan. He re-materialized, still wearing it, in 2369. (TNG: "Relics")
When Captain Jean-Luc Picard visited Mintaka III one last time, he had his arm in a sling due to his prior injury on the planet. (TNG: "Who Watches The Watchers")
While on the Teplan homeworld, Doctor Julian Bashir cured the broken arm of a Teplan boy who wore a sling. (DS9: "The Quickening")
When Kathryn Janeway cared for Q , she placed his arm in a sling. (VOY: "The Q and the Grey")
In 2374, Chief Miles O'Brien beamed up two survivors from the USS Honshu. One was wearing a sling. (DS9: "Waltz")