(as) thick as thieves

(as) thick as thieves

Having a close, intimate friendship or alliance. Anna and Beth are together all the time these days—they're as thick as thieves. The guys who work in the warehouse are thick as thieves. They don't really socialize with anyone else in the company.
See also: thick, thief
Farlex Dictionary of Idioms.

*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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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 This term uses thick in the sense of "intimate," a usage that is obsolete except in this simile.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
The American Heritage® Dictionary of Idioms by Christine Ammer.

thick as thieves

If two or more people are as thick as thieves, they are very friendly with each other. Jones and Cook had met at the age of ten and were as thick as thieves. Grant went to school with Maloney, the other lawyer in town. They're thick as thieves.
See also: thick, thief
Collins COBUILD Idioms Dictionary, 3rd ed.

thick as thieves

(of two or more people) very close or friendly; sharing secrets. informal
See also: thick, thief
Farlex Partner Idioms Dictionary

(as) thick as ˈthieves (with somebody)

(informal) (of two or more people) very friendly with each other, especially in a way that makes other people suspicious: Those two are as thick as thieves — they go everywhere together. OPPOSITE: be at daggers drawn
See also: thick, thief
Farlex Partner Idioms Dictionary

thick as thieves

On intimate terms; very good friends. The use of thick for “intimate” survives mainly in this cliché, which no doubt owes its popularity to alliteration. It was already proverbial, according to Theodore E. Hook, in 1833 (The Parson’s Daughter): “She and my wife are as thick as thieves, as the proverb goes.”
See also: thick, thief
The Dictionary of Clichés by Christine Ammer
See also:
  • thick as thieves
  • thief
  • be (as) thick as thieves
  • have intimate relations with
  • have intimate relations with (one)
  • close as the bark to the tree
  • intimate to
  • intimate to (someone)
  • emotional affair
  • finger as