An Etymological Dictionary of the German Language/Annotated/gewiß
gewiß, adjective and adverb, ‘sure(ly), certain(ly), confident(ly),’ from the equivalent Middle High German gewis(ss), adjective, gewisse, adverb, Old High German gewis(ss), adjective, gewisso, adverb, ‘certain, sure, reliable’; corresponding to Dutch wis, gewis; Goth only in unwisa- (misspelt for *unwissa), ‘uncertain.’ The Old Teutonic wissa (gawissa-) is an old participle of the Gothic pret.-pres. witan, Old High German wiȥȥan (see wissen), from witta-, widto- (allied to the Aryan root vid). With regard to the pregnant meaning, ‘what is certainly known,’ for ‘what is known,’ compare laut, literally ‘what is heard.’