An Etymological Dictionary of the German Language/Annotated/knacken

by Friedrich Kluge

knacken, verb, ‘to crack,’ from Middle High German knacken, gnacken, ‘to split, crack,’ wanting in Old High German; English knack, Middle English cnak, ‘crack,’ Icelandic knakkr; Modern High German Knack, ‘crack,’ Middle High German not yet found. To the same root Old Icelandic knoka, Anglo-Saxon cnocian, English to knock, formed by gradation, seem to belong. The words are based on an imitative root which is peculiar to Teutonic.