wasted money; idle money; dead money
Refers to money that is sitting unused, not earning interest or being put to productive use. Often used in personal finance or investment contexts.
Let's put that idle money sitting in the bank into investments.
Cash stashed in a drawer is dead money.
Compound of 死に (continuative form of 死ぬ, 'to die') and 金 ('money'). Literally 'dead money', referring to money that is not 'alive' or productive.