be (as) thick as thieves

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be (as) thick as thieves

To be very close friends. Anna and Beth are together all the time these days—they're as thick as thieves.
See also: thick, thief
References in periodicals archive ?
The mates were thick as thieves at a launch party for Sony Ericsson at Baker Street in London on Tuesday.
Madonna and Rupert Everett were thick as thieves," DeCaro says.