additional payment; extra money paid on top
Refers to money paid in addition to an agreed amount, often to cover a shortfall or as a supplement. Can be used in contexts like settling a bill, making up a difference, or adding a tip.
The fare wasn't enough, so I paid extra money.
He paid the bill with additional money on top.
Compound of 追い (oi, 'chasing' or 'adding') and 銭 (sen, 'money'). The exact historical derivation is uncertain, but it likely arose from the idea of 'chasing after' a payment with additional coins.