An Etymological Dictionary of the German Language/Annotated/sonst

by Friedrich Kluge

sonst, adverb, ‘else, otherwise, formerly,’ from Middle High German sunst, sust, earlier Middle High German and Old High German sus, ‘thus’ (the change in meaning from ‘thus’ to ‘else’ is generally explained by the ellipse of a negative particle). Old High German and Old Saxon sus, Dutch zus, ‘thus,’ seem to be of the same stem as Old High German and Middle High German sô.