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
Baldwin	BAWLD-win (English)	English, Ancient Germanic
Stirling	STUR-ling	English (Rare)
Timo (1)	TEE-mo	Finnish, German, Dutch
DİLŞAd	-	Turkish
Harshada	-	Indian, Hindi, Marathi
Maite (2)	-	Basque