What is the origin of the idiom "thick as thieves" and when to use it?

The origins of the phrase can be traced back to the early 19th century. It is believed to have originated in the criminal underworld of London, England, where thieves and pickpockets would form close alliances to carry out their illicit activities. The phrase likely refers to the notion that these criminals were so close that they could be compared to family members, or even thicker than blood. The phrase became more widely known in the mid-20th century and is now used in a variety of contexts to describe close friendships or alliances.