It's long been recognized in legal circles that money is fungible. If you don't put in specific fire-walled mechanisms, it can be used for any purpose whatsoever. I don't think the NRA has much of a defense there.
It's a fun word. Basically it means that a dollar in your left pocket is fully interchangeable with a dollar in your right pocket. So if you can't buy food with left pocket dollars, but you can buy gas, you won't have to spend a right pocket dollar on gas and you can use it to buy the sandwich. But if you only have that right pocket dollar, you can't have both. You could afford only food or gas.
Having that left pocket dollar allowed you to buy something it couldn't be used for directly.
Definition of fungible
1: being something (such as money or a commodity) of such a nature that one part or quantity may be replaced by another equal part or quantity in paying a debt or settling an account
- Oil, wheat, and lumber are fungible commodities. - fungible goods
2: capable of mutual substitution :
interchangeable
- … the court's postulate that male and female jurors must be regarded as fungible —George Will
3: readily changeable to adapt to new situations :
flexible
- Managers typically use more than a hundred different lineups over the course of the season. Batting orders are so fungible that few players last long in one spot. —Tom Verducci
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fungibility
play \ˌfən-jə-ˈbi-lə-tē\ noun