An Etymological Dictionary of the German Language/Annotated/Knopf
Knopf, masculine, ‘button, knob, pommel,’ from Middle High German and Old High German knopf, masculine, ‘ protuberance on plants, bud, pommel of a sword, knot, loop’; compare Anglo-Saxon *cnopp, masculine, English knop, ‘button, bud’; Dutch knop, ‘bud, button, knot on plants.’ Gothic *knuppa- is wanting; under Knauf its graded form Gothic *knaupa- was assumed, which would represent *knauppa-, for the stem loses its final b, as is shown in Middle High German knübel, masculine, ‘knuckle,’ as well as Anglo-Saxon *cnobba, Middle English knobbe, English knob; compare also Modern Dutch knobbel, masculine, ‘knot, bulb, weal,’ and High German Knubbe. Besides the words hitherto adduced, from which we may infer an old u root (compare especially Knauf), there are some abnormal forms, Old Icelandic knappr, ‘button, pommel,’ Anglo-Saxon cnœp, Middle English knap. Compare Knospe, Knubbe, knüpfen, and Knüppel.