…
What he was was a bogeyman. Smoke and a whisper, nothing more. If information was hard currency, the best we had on Popov was an IOU. Nobody ever laid a finger on him, and that was because he didn’t exist.”
“So how come,” River began, and stopped himself. Asking questions is good. An early lesson. If you don’t know something, ask. But before you ask, try to work it out for yourself. He said, “So the smoke and the whispers were deliberate. He was invented to have us chasing someone who wasn’t there.”
The O.B. nodded approval. “He was a fictional spymaster, running his own fictional network. It was meant to have us tying ourselves in knots. We did something similar in the war. Operation Mincemeat. And one of the lessons we learned from that was you can discover an awful lot from the details you’re asked to believe.
— Dead Lions (Mick Herron), Loc. 1163–9
An IOU stands for “I owe you” and is usually an informal written acknowledgment of debt between two parties. It typically shows the debtor and the amount owed, and it may be signed to confirm the acknowledgement.
An IOU (abbreviated from the phrase “I owe you”)