Dictionary.com
definitions

IOU

or 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.

Origin of IOU

First recorded in 1610–20; representing I owe you
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for i.o.u.

voucher, bond, I.O.U.
British Dictionary definitions for i.o.u.

IOU

noun
  1. a written promise or reminder to pay a debt

Word Origin for IOU

C17: representing I owe you
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for i.o.u.

I.O.U.

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

Online Etymology Dictionary, © 2010 Douglas Harper
i.o.u. in Culture

IOU

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

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.