IOU (???-yo?o?)n. A promise to pay a debt, especially a signed paper stating the specific amount owed and often bearing the letters IOU.[From the pronunciation of I owe you.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.IOU n a written promise or reminder to pay a debt [C17: representing I owe you]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014IOU or I.O.U., n., pl. IOUs, IOU’s, or I.O.U.’s. a written acknowledgment of a debt, esp. an informal one consisting only of the letters IOU, the sum owed, and the debtor’s signature. [1785?95; representing I owe you] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.