as thick as thieves
informal phrase

- if two people are as thick as thieves, they are very friendly with each other and seem to share a lot of secrets, making other people think they are hiding or planning something

Longman Dictionary

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ORIGIN

In the 18th century ‘thick’ was used to mean ‘closely allied with’ as in this example from Richard Twining’s “Selected Papers of the Twining Family, 1781:

“Mr. Pacchicrotti was at Spa. He and I were quite ‘thick.’ We rode together frequently. He drank tea with me.”

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THE SECRET LANGUAGE OF THIEVES

The association of thieves with conspiratorial and secretive language was well established in England in the 17th and 18th centuries. Many of those on the fringes of society used secret words and phrases to converse amongst themselves.

Backslang was one example of this, the best known survival of backslang being ‘yob’ for ‘boy’.

A popular dictionary of slang from 1698 explained how to decipher the language of “the tribes of gypsies, beggars, thieves, cheats etc.” so that law-abiding citizens could “secure their money and preserve their lives”.

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SYNONYMS
for as thick as thieves

to be inseparable, to stick together like glue, to be like peas in a pod

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SMUGGLE OWAD into a conversation, say something like:

“Be careful what you say to either Jim or Jeremy, those two are AS THICK AS THIEVES.”