Letters on a debtor's slip

• Letters to a creditor
• "Got you next time" letters
• Written reminder of debt
• Pot note
• Note in a poker game, e.g.
• Piece of paper you wouldn't need if both people had Venmo
• Letters indicating a promise to pay a debt
• Poker player's last resort
• Last poker bet, sometimes

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 ...