Incapable of being bought back.

不能赎回的。

Not exchangeable for gold or silver: said of paper money.

不能兑换的(指钞票而言)。

Irreclaimable; thoroughly depraved.

不能矫正的；无可救药的。

A bond that cannot be redeemed before maturity.

定期偿还公债(到期前不能偿还的公债)。