An Etymological Dictionary of the German Language/Annotated/schlicht
schlicht, adjective, ‘plain, homely, honest,’ Modern High German only, formed to represent the meanings of Middle High German slëht (see schlecht), which became obsolete in Modern High German schlecht, from the Middle High German and Old High German verb slihten, ‘to make plain, smooth over,’ and the Middle High German abstract form slihte, feminine, ‘straightforwardness’; compare Old High German slihten, ‘to make plain,’ slihtî, allied to slëht, ‘straight, even.’