Among other figurative meanings, the adjective thick has the sense of close in confidence and association, intimate, familiar. In Literary Anecdotes of the Eighteenth Century, published in 1812, John Nichols quoted Edmund Law (1703-87), Bishop of Carlisle:

“Yes,” said he, “we begin now, though contrary to my expectation, and without my seeking, to be pretty thick; and I thank God, who reconciles me to my adversaries.”

The Scottish poet and novelist Walter Scott (1771-1832) used the phrase as thick as three in a bed in The Monastery (1820):

“That’s right, Captain,” vociferated David; “you twa will be as thick as three in a bed an’ ance ye forgather.”

But this phrase was already proverbial when Francis Asbury wrote in his journal, on Thursday 1^st July 1784:

Arriving at a small house, and halting for the night, we had, literally, to lie as thick as three in a bed.

(Francis Asbury (1745-1816), an Englishman, was one of the first two bishops of the Methodist Episcopal Church in the United States.)

Another phrase was as thick as inkle-weavers (the word inkle designates a kind of linen tape formerly used to make laces, or the linen yarn from which this is manufactured). It is first recorded, but in a different form, in A New Dictionary of the Terms Ancient and Modern of the Canting Crew (1699), by “B. E. Gent.”:

Inkle: Tape. As great as two Inkle-makers, or as great as Cup and Cann.

In A Complete Collection of Genteel and Ingenious Conversation, a satire on the use of clichés written in the first decade of the 18^th century but published in 1738, the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745) also used the phrase in this form:

She and you were as great as two Inkle-weavers. I’ve seen her hug you, as the Devil hug’d the Witch.

An explanation was given in Notes and Queries of 12^th August 1865:

Inkle, or beggar’s inkle, is a kind of coarse tape used by cooks to secure meat previous to being spitted, and farriers to tie round horses’ feet, &c. The introduction of this kind of inferior tape was from the Low Countries, during the persecutions of the sixteenth century. The traffic was carried on by a few foreign weavers, who kept the secret among themselves, and being of one trade, language, and religion, they of course became staunch familiar friends; or, as Burns describes his twa dogs, “Unco pack and thick together.” Hence it is now said of persons very friendly, “They are as thick as inkle-weavers.”

One of the earliest attestations of as thick as thieves is in The Parson’s Daughter (1833), by the English writer Theodore Edward Hook (1788-1841)—but the phrase is referred to as proverbial:

“That,” said Thompson, “is the young lady I saw here after church yesterday.”
“Exactly,” said the Squire. “She and my wife are as 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.”

This English phrase is similar to:

– Latin intelligunt se mutuo, ut fures in nundinis,

– French s’entendre comme larrons en foire,

– Italian intendersi come i tiraborse alla fiera,

– German sie verstehen einander wie Diebe beim Jahrmarkt,

which all mean, literally, to get on like thieves at a fair.

In Spanish, the same idea is conveyed by entenderse como lobos de la misma camada, to get on like wolves from the same litter.