Monopoly Money Meaning

(idiomatic) Money that doesn't really exist, referring to fraudulent record keeping.

Example: Play money printed on paper, especially differently colored denominations of the board game Monopoly.
2005, Craig M. Boise, Playing with ‘Monopoly Money’: Phony Profits, Fraud Penalties and Equity, Minnesota Law Review, volume 90, page 144
  They did so to conceal the fact that they were playing with Monopoly money—fabricating profits as phony as the pastel-colored money used in the classic Parker Brothers board game.