Cooper

Surplus, hidden or additional quantities of something, especially if secretly concealed until a key moment for advantageous gain or cheating

Origin: random hyper-localised sayings, Rothwell, Leeds 1990s.

Person A: Yay!, I’ve won this game of Monopoly.
Person B: Ahh, no, because I have this COOPER MONEY!

or

I thought we'd run out of crisps, but I found all these cooper crisps in the other cupboard.