I'll forge a rubber check for you
The mean thing you say when someone asks for money for something you already gave them. You’re saying you’ll fake a check and even fake the writing on it, because they’re totally ridiculous.
You: 'I’ll forge a rubber check for you!' (after your kid asked for $10 for a snack you already gave them.)
You: 'I’ll forge a rubber check for you!' (because your neighbor wants cash for a plant you already tossed.)
You: 'I’ll forge a rubber check for you!' (when your friend asked for a $5 tip for a soda you already drank.)