'IOU' definitions:

Definition of 'IOU'

From: WordNet
noun
An informal debt instrument; representing `I owe you'

Definition of 'IOU'

From: GCIDE
  • I O U \I O U\, IOU \I.O.U\([imac]" [=o]" [=u]"). [phonetic, from I owe you.] 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. --Wharton. --Story. [1913 Webster]