Definition from Wiktionary, the free dictionary
From the pronunciation of "I owe you", not the initial letters.
- I owe you.
- Customarily, if you borrow $10 for lunch from a coworker, writing IOU $10 on a yellow sticky note acts as a legal promissory note that you will repay said amount on payday.
IOU (plural IOUs)
- An informal written note promising to pay the stated sum.
- When David couldn’t afford to match the money in the poker pot, he added an IOU for the remaining amount instead.