It's a proverb.
Matteux (1712). Quixote
. "The old proverb still holds good, thieves are never rogues among themselves."
There is honor among thieves.
Entre bueyes no hay cornadas
"There is honor among thieves"
~ English proverb
We had our specialties in this contest of wits. One was distinguished as a sleuth. He fed on detective mysteries as a cat on a chicken-bone. He thought them out by day and dreamed them out by night, to the great exasperation of the official detectives, with whom their solution was a com- mercial, not in the least an intellectual, affair. They solved them on the plane of the proverbial lack of honor among thieves, by the formula, "You scratch my back, and I’ll scratch yours."
~Jacob A. Riis (1849–1914). The Making of an American. 1901.
IX. Life in Mulberry Street
All is fair in war and politics. It is a game of force and fraud. There is said to be honor among thieves, but one does not look for such a thing among statesmen.
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The trouble with writing a book about yourself is that you can't fool around. If you write about someone else, you can stretch the truth from here to Finland. If you write about yourself the slightest deviation makes you realize instantly that there may be honor among thieves, but you are just a dirty liar.