An Etymological Dictionary of the German Language/Annotated/klopfen
klopfen, verb, ‘to knock,’ from Middle High German klopfen, Old High German chlopfôn, weak verb, ‘to knock, rap’; Gothic *kluppôn is not warranted by corresponding forms in the other Old Teutonic dialects; compare also Dutch kloppen. Further, Old High German chlocchôn, Middle High German klocken, ‘to knock,’ which are not indubitably allied to klopfen. With the latter the cognates discussed under Klaff are connected by gradation, and these point to a Gothic *klappôn, ‘to strike.’