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