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.

Name	Pronounced	Usage
Caradoc	-	Welsh
Takashi	tah-kah-shee	Japanese
Sithembile	-	Southern African, Zulu
Odell	o-DEL	English
Niloufar	-	Persian
Svjetlana	-	Croatian, Serbian