dunsel
Something (especially part of a vessel) that is useless, or superfluous or unnecessary.
If I'm not part of the crew and earn my keep, then I'm a dunsel. And you'll not have a dunsel aboard long […]
A captain of a vessel who has little or no practical use.
McCoy: “Who the blazes is ‘Captain Dunsel’?” / Spock: “Dunsel, Doctor, is a term used by midshipmen at Starfleet Academy. It refers to a part which serves no useful purpose.”
Where it comes from
Possible coinage as a variation of English dunnage as dun + -sel with the suffix indicating singulative or diminutive. Earliest so-far documented use was in a Star Trek episode, "The Ultimate Computer". Was coined by a writer for the original "Star Trek" series (Episode name: The Ultimate Computer.").
Adapted from the Wiktionary entry dunsel (CC BY-SA 4.0).