Green’s Dictionary of Slang

1. (US Und.) a bad cheque.

1948		D. Burley N.Y. Amsterdam News 10 Jan. 17: A ‘Frog’ is a bad check.
1967		J. Breslin World of Jimmy Breslin (1968) 117: A transit item [i.e. a cheque] returned unpaid is known as a ‘bum map’ or a ‘frog.’.

2. see frogskin n.¹