An Etymological Dictionary of the German Language/gewiß
gewiß, adj. and adv., ‘sure(ly), certain(ly), confident(ly),’ from the equiv. MidHG. gewis(ss), adj., gewisse, adv., OHG. gewis(ss), adj., gewisso, adv., ‘certain, sure, reliable’; corresponding to Du. wis, gewis; Goth only in unwisa- (misspelt for *unwissa), ‘uncertain.’ The OTeut. wissa (gawissa-) is an old partic. of the Goth. pret.-pres. witan, OHG. 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,’ comp. laut, lit. ‘what is heard.’