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
Zilpah	ZIL-pə (English)	Biblical, Biblical Hebrew
Madhava	-	Sanskrit, Hinduism
SÉAfra	-	Irish
Hardy	HAHR-dee	English
Andro	-	Croatian, Georgian
Suharto	soo-HAHR-to	Indonesian, Javanese