i.o.u.

1. a written acknowledgment of a debt, especially an informal one consisting only of the letters IOU, the sum owed, and the debtor’s signature.

noun

1. a written promise or reminder to pay a debt

also IOU, I O U, 1610s, originally as IOV (see V); with punning reference to “I Owe You.”

Abbreviation for “I owe you.” An IOU is a written statement of a borrower’s obligation to pay a debt.