1. 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.

Noun

1. 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.