Definition: Very close friends.
For example: Those two are very close. They are as thick as thieves.
Thick As Thieves (1999) – movie
Two self-styled criminal masterminds find themselves in a turf battle neither much cares about in this underworld story that balances comedy against drama. Alec Baldwin plays Mackin, a career thief who picks his jobs shrewdly and carefully, and prefers to spend his downtime with his collection of rare jazz LP’s and looking after his dog. Pointy (Michael Jai White) is a young upstart gangster trying to develop a taste for refinement and the good life. When Pointy sets up Mackin, Mackin is forced to retaliate, and before long both men and their associates are in the middle of a war neither is especially interested in winning, which begins to escalate in comic fashion. The skirmish eventually attracts the attention of a female cop (Rebecca De Mornay) who’s become interested in Mackin’s method of operation. Thick As Thieves received its world premiere at the 1999 Sundance Film Festival. ~ Mark Deming, Rovi