idiombe (as) thick as thievesused in order to say that two people or groups are very friendly and share a lot of secrets, especially when you are annoyed by this or disapprove of it:Nick and Auntie Lou were as thick as thieves, and often sat in the kitchen giggling about something.Everyone knows Labour and the unions are as thick as thieves.
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.