From Longman Dictionary of Contemporary Englishbe (as) thick as thievesbe (as) thick as thievesFRIENDLYif two people are as thick as thieves, they are very friendly with each other and seem to share a lot of secrets, making other people think they are hiding or planning something Lately Nick and Lou have been as thick as thieves. → thick
Examples from the Corpus
be (as) thick as thieves• Then they'd be as thick as thieves again.• All among the trees, the birds are thick as thieves and noisy as conventioneers. 