The cliché thick as thieves, means that two or more people have a very close relationship with one another and are intimately allied. It sometimes also implies that two or more people are involved in some sort of conspiracy. In other words, they are in cahoots with one another.
On Tuesday, June 16, 1809, the Telegraph newspaper in Nashua, New Hampshire ran a story by Meredith Nicholson, author of “The House of a Thousand Candles.” The story was “The Port of Missing Men” and made use of the phrase “thick of thieves.”
The old man leaned upon the table heavily.
“That amiable Francis” —
“The suggestion is not dismaying. Francis would not know an opportunity if it offered.”
“But his mother — she is the devil!” blurted the old man.
“Pray drop that,” said Armitage in a tone that caused the old man to look at him with a new scrutiny. “I want the paper back for the very reason that it contains that awful indictment of her. I have been uncomfortable ever since I gave it to you, and I came to ask you for it that I might keep it safe in my own hands. But the document is lost. Am I to understand that Francis has it?”
“Not yet. But Rambaud has it, and Rambaud and Francis are as thick as thieves.”
“I don’t know Rambaud. The name is unfamiliar.”
“He has a dozen names — one for every capital. He even operated in Washington, I have heard. He’s a blackmailer who aims high — a broker in secrets, a scandal peddler. He’s a bad lot, I tell you. I’ve had my best men after him, and they’ve just been here to report another failure. If you have nothing better to do” — began the old man.
In the book “The Parson’s Daughter” written by Theodore E. Hook and published by Carey, Lea and Blanchard in 1833, the author wrote this on page 184:
“Exactly,” said the Squire. “She and my wife are thick as thieves, as the proverb goes: they know each other’s secrets, and lay their heads together, to do all the mischief they can. However, it would be a great match for her if it was brought about. He is a good fellow, and she is a good girl.”
The phrase “thick as thieves” was actually a translation of the French idiom “s’entendre comme larron en foire” which in English means “like thieves at a fair.” The French phrase means to be complicit with another in an activity which may or may not be lawful.
French novelist and playwright Honoré de Balzac (20 May 1799 – 18 August 1850) published the short story “The Three Clerks of Saint Nicholas” in his larger work “Droll Stories — Volume 2” in which the following is found:
The host bustled about, turned the spits, and prepared a glorious repast, for these three dodgers, who had already made noise enough for a hundred crowns, and who most certainly would not even have given up the copper coins which one of them was jingling in his pocket. But if they were hard up for money they did not want for ingenuity, and all three arranged to play their parts like thieves at a fair.
The French phrase comes from the Latin proverb: Intelligunt se mutuo, ut fures in nundinis, which translates into English as “A thief knows a thief, as a wolf knows a wolf.”
Based on the Latin proverb, individuals being thick as thieves is something the world has known about for centuries.
Historically Speaking 