thick as thieves

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*thick as thieves

Cliché very close-knit; friendly; allied. (Thick = close and loyal. *Also: as ~.) Mary, Tom, and Sally are as thick as thieves. They go everywhere together. Those two families are thick as thieves.
See also: thick, thief

thick as thieves

Intimate, closely allied, as in The sisters-in-law are thick as thieves. This term uses thick in the sense of "intimate," a usage that is obsolete except in this simile. [Early 1800s]
See also: thick, thief