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. 2019
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
also IOU, I O U, 1610s, originally as IOV (see V); with punning reference to "I Owe You."
Online Etymology Dictionary, © 2010 Douglas Harper
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.