An Etymological Dictionary of the German Language/klopfen
klopfen, vb., ‘to knock,’ from MidHG. klopfen, OHG. chlopfôn, wk. vb., ‘to knock, rap’; Goth. *kluppôn is not warranted by corresponding forms in the other OTeut. dials.; comp. also Du. kloppen. Further, OHG. chlocchôn, MidHG. 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 Goth. *klappôn, ‘to strike.’