"money," c.1918, American English slang, perhaps from smack (v.1) on notion of something "smacked" into the palm of the hand. Extended form smackeroo is attested from 1939.
[the money sense, attested also of pesos and pounds sterling, may echo the slapping down of a bill on a counter, gambling table, etc, and hence be semantically related to plank and plunk for coins]