An Etymological Dictionary of the German Language/Annotated/wohl
wohl, adverb, ‘well, probably,’ from the equivalent Middle High German wol, Old High German wola (earlier wëla), adverb from gut; corresponding to Old Saxon wē̆l, Dutch wel, Anglo-Saxon wē̆l, English well, Gothic waila. The primary meaning of this common Teutonic adverb is ‘as one could wish,’ because it is derived from the root of wollen. Aryan welo-, ‘wish, desire,’ is also indicated by Sanscrit vára, masculine and neuter, ‘wish, desire,’ to which váram ã (or práti váram), ‘as one could wish, as one likes,’ is allied.