Thick as Thieves

This another expression I found.
It's actually the name of a film, which I haven't seen yet. In fact, I had never heard of it until a month ago or so. The thing was like this. My brother in law started to tell me about this movie he had seen, and when I asked him for the name of the movie he mumbled "The Code" and this little phrase. Of course I needed to know what its meaning was, and here it is: Cliché very close-knit; friendly; allied. It makes reference to the closeness and intedepence existing in gangs of thieves. The word thick is used in its sense of 'densely arranged' as in thick hair, grass (from: http://users.tinyonline.co.uk/gswithenbank/sayingst.htm).
I don't see myself using this phrase, but I found the combination of words interesting. I think it shows how creative we can be with the language. Everytime I come across these idiomatic expressions, in Spanish or English, I wonder who said it for the first time, and how it became popular. I think that this phrases are legacies from other times. Someone sharing a little piece of their present with us. Some become outdated, and lose popularity, but it is funny tosee how language is recycled. New generations take older expressions, assign new meanings or not to them, and bring them back to life, bridging the gaps between generations.