An Etymological Dictionary of the German Language/niesen
niesen, vb., ‘to sneeze,’ from the equiv. MidHG. niesen, OHG. niosan, str. vb.; corresponding to Du. niezen, OIc. hnjósa (to which hnore, m., ‘sneezing,’ is allied), MidE. nêsin; also AS. fneôsan, MidE. fnêsen, equiv. to Du. fniezen, ‘to sneeze.’ The two Teut. roots hnus and fnus seem to have been orig. identical; with these are connected MidE. snêsen, E. to sneeze. The pre-Teut. root qnus, ksnus, may be onomatopoetic.