1
general::
Phrase(s): *thick as thieves
Cliché very close-knit; friendly; allied. (Thick = close and loyal. *Also: as ~.) • Mary, Tom, and Sally are as thick as thieves. They go everywhere together. • Those two families are thick as thieves.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Intimate, closely allied, as in The sisters-in-law are thick as thieves. This term uses thick in the sense of ‘‘intimate," a usage that is obsolete except in this simile. [Early 1800s]
American Heritage Idioms