An Etymological Dictionary of the German Language/Knopf
Knopf, m., ‘button, knob, pommel,’ from MidHG. and OHG. knopf, m., ‘ protuberance on plants, bud, pommel of a sword, knot, loop’; comp. AS. *cnopp, m., E. knop, ‘button, bud’; Du. knop, ‘bud, button, knot on plants.’ Goth. *knuppa- is wanting; under Knauf its graded form Goth. *knaupa- was assumed, which would represent *knauppa-, for the stem loses its final b, as is shown in MidHG. knübel, m., ‘knuckle,’ as well as AS. *cnobba, MidE. knobbe, E. knob; comp. also ModDu. knobbel, m., ‘knot, bulb, weal,’ and HG. Knubbe. Besides the words hitherto adduced, from which we may infer an old u root (comp. especially Knauf), there are some abnormal forms, OIc. knappr, ‘button, pommel,’ AS. cnœp, MidE. knap. Comp. Knospe, Knubbe, knüpfen, and Knüppel.