Английский Этимологический словарь - smack (2)

- "make a sharp noise with the lips," 1557, probably of imitative origin. The sense of "slap" first attested 1746, perhaps influenced by Low Ger. smacken "to strike, throw." Smacker "money" is U.S. slang, c.1918, perhaps from notion of something smacked into the palm of the hand.