The term was used as an insult (albeit a playful one) to Captain James T. Kirk during the 2268 war games test of the M-5 Multitronic Unit created by Dr. Richard Daystrom. Following the computer's successful maneuvers, Commodore Bob Wesley called Kirk "Captain Dunsel", since the M-5's ultimate goal was to eliminate the need for living captains, thereby rendering Kirk useless.
Wesley's remark was a source of confusion for Dr. Leonard McCoy, who was unfamiliar with the term. After initially asking Kirk, who ignored his question, he then inquired of First Officer Spock. The Vulcan explained Wesley's comment only after Kirk had left the bridge, obviously stung by the commodore's insult. (TOS: "The Ultimate Computer")