play with house money

1. To fund something with sudden winnings or unexpected earnings, thus making such an endeavor seem less risky. A: "Who cares if our trip gets canceled and we don't get our deposit back? We're using that money I won, so we're playing with house money!" B: "No, we're using real money, money that I would ideally not like to lose."

2. By extension, to be in a situation that has no significant downside or loss, no matter the outcome. Regardless of what happens tonight, the Owls have had a great playoff run and far exceeded expectations, so I say they're playing with house money going into this elimination game.