The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with funny money
Counterfeit money; also, money from an obscure or questionable source. For example, The police warned storekeepers that some funny money was being passed around town. This expression probably endures because of its rhyme.