As thick as thieves is an interesting idiom that first appeared at the end of the eighteenth century. We’ll look at the meaning of the phrase as thick as thieves, its origin, and it’s used in a few example sentences.
As thick as thieves describes a relationship between people that is very close. When people are as thick as thieves they generally share confidences, spend a great deal of time together and are great friends. Originally, the term was as thick as two thieves, as it generally describes a relationship between two people. The word thick as used in the idiom as thick as two thieves means intimate, familiar, closely allied, sharing confidences. The word thieves conjures up the connotation of two people who conspire together in secret, two people whose association is close and excludes the general population. By 1833 the phrase as thick as thieves was referred to as a proverb, intimating that it was well known in spoken English. Just how long the idiom existed in spoken English before it made its way into written English is unknown.
In the Spotlight programme Mr Cushnahan also implied he had influence over senior Nama official Ronnie Hanna, a former Head of Assets, telling Mr Miskelly the two men were “as thick as thieves”. (The Irish Times)
They were as thick as thieves, even if right now Rocco believes swinging London is where his heart is, as opposed to wretched, tedious New York. (The Independent)
“The foal is looking great and, with it being the second to be born here in the space of just a week, we’re sure the two new arrivals will be as thick as thieves.” (The Liverpool Echo)