Coventry

 Looking at "Coventry Carol," for Advent I wondered what exactly it was.  I knew of the phrase sent to Coventry but discerned that must be later for some historical reaction to an already existing place. This part I had right. But I guessed that "coventry" as a word, and thus a place-name, must be related to covenant, convent, coven, and the like. 

Wrongo! This is dismissed as a "folk etymology," one of the worst insults that can be bestowed on a guess.

 city in Warwickshire, mid-13c., an alteration of Old English Couentre (1043), probably literally "Cofa's tree," from Old English masc. personal name Cofa (genitive Cofan) + tree (n.). If this is correct, the name might refer to a boundary marker or a public assembly place. The explanation that it was named for a convent founded there 11c. likely would be folk etymology.

My wife knew that being sent to Coventry meant to be excluded and not spoken to or listened to, which I had only vaguely understood.  Looking it up, it was used by military officers to socially sanction others who had committed dishonourable acts that did not rise to the level of courts martial.