The memorandum of a debt given by the borrower to the lender. It must not contain a promise to pay. The letters mean, “I owe You.”
An I.O.U. requires no stamp, unless it specifies a day of payment, when it becomes a bill, and must have a stamp.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on I O U from Infoplease: