Catgut was a type of suture material made from the intestinal fibers of animals.
In 2268, when trying to make a point about the foolishness of wishing away scientific advancements such as interstellar flight, James T. Kirk compared it to Leonard McCoy wishing he still operated with scalpels and sewed up patients with catgut like his great-great-great-great-grandfather used to do. (TOS: "Return to Tomorrow")