An Etymological Dictionary of the German Language/sonst

by Friedrich Kluge

sonst, adv., ‘else, otherwise, formerly,’ from MidHG. sunst, sust, earlier MidHG. and OHG. sus, ‘thus’ (the change in meaning from ‘thus’ to ‘else’ is generally explained by the ellipse of a negative particle). OHG. and OSax. sus, Du. zus, ‘thus,’ seem to be of the same stem as OHG. and MidHG. sô.