Letters for one in the red

• "Will pay later" note
• Note written by a shy person?
• Evidence of shyness?
• Gambling tab
• Letters of payback
• Bad bettor's letters
• Letters before a monetary amount
• Borrower's letters

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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 ...

Wikipedia Word definitions in Wikipedia
An IOU (abbreviated from the phrase "I owe you") is usually an informal document acknowledging debt . An IOU differs from a promissory note in that an IOU is not a negotiable instrument and does not specify repayment terms such as the time of repayment. ...