An Etymological Dictionary of the German Language/knacken

by Friedrich Kluge

knacken, vb., ‘to crack,’ from MidHG. knacken, gnacken, ‘to split, crack,’ wanting in OHG.; E. knack, MidE. cnak, ‘crack,’ Ic. knakkr; ModHG. Knack, ‘crack,’ MidHG. not yet found. To the same root OIc. knoka, AS. cnocian, E. to knock, formed by gradation, seem to belong. The words are based on an imitative root which is peculiar to Teut.