Definitions of I O U

  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 Dictionary DB
  2. Designed to be a contr. of the sentence or phrase. "I owe you"; three capital letters written on a slip of paper, under which a given sum and a signature is put-being a brief acknowledgment of money lent. Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.