Dunsel

From Memory Alpha, the free Star Trek reference.

Dunsel is a term used by mid-shipmen in the 23rd century to describe a part which serves no useful purpose.

The term was used as an insult (albeit a playful one) to Captain James T. Kirk during the war games test of the M-5 Multitronic Unit created by Dr. Richard Daystrom. Commodore Robert Wesley called Kirk Captain Dunsel, to the confusion of Dr. Leonard McCoy. Kirk's First Officer Spock explained the term only after Kirk had left the bridge, stung by the insult. (TOS: "The Ultimate Computer")

McCoy's ignorance of the term suggests that unlike Kirk, McCoy did not graduate from Starfleet Academy but rather received a direct commissioning appointment to Starfleet. This is consistent with the way many staff officers (physicians, lawyers, chaplains) join the U.S armed services today. Or perhaps McCoy wasn't aware of the jargon of the day whereas Wesley and Kirk were.