(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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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 This term uses thick in the sense of "intimate," a usage that is obsolete except in this simile.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]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
thick as thieves
(of two or more people) very close or friendly; sharing secrets. informalFarlex Partner Idioms Dictionary © Farlex 2017
(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 drawnFarlex Partner Idioms Dictionary © Farlex 2017
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.”
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer