Fromun- +‎embellished.

unembellished (comparativemoreunembellished,superlativemostunembellished)

1. Plain,unadorned, orsimple.

   Synonyms:austere,bare,modest,undecorated,unadorned;see alsoThesaurus:unadorned

   Antonyms:adorned,decorate,complicated;see alsoThesaurus:gaudy

   • 1941 October, C. Hamilton Ellis, Charles E. Lee, “The Welsh Highland Railway—III”, inRailway Magazine, page435:

     Thus passed the last of the North Wales Narrow Gauge locomotives, and one of the few American-built locomotives to survive in Great Britain; the sadly cropped though once elegantRussell, and theunembellished yet useful Yankee plug-ugly.