I O U

(ī´ ō´ ū´)
1.A paper having on it these letters, with a sum named, and duly signed; - in use in England as an acknowledgment of a debt, and taken as evidence thereof, but not amounting to a promissory note; a due bill.
Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.