An Etymological Dictionary of the German Language/Annotated/schal

by Friedrich Kluge

schal, adjective, ‘hollow, stale, flat,’ from Middle High German (rare) schal, adjective, ‘turbid,’ to which Middle High German verschaln and schaln, ‘to become dim’; compare Dutch verschalen, ‘to get flat or stale,’ English shallow. The term, the origin of which is obscure, is wanting in the Upper German dialects.