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
Irmentrud	EER-men-troot	German (Archaic)
Baird	[bɛəd]
EusÉBio	-	Portuguese
Remo	RE-mo	Italian
Nikolina	-	Bulgarian, Croatian, Serbian, Macedonian
Fernande	fer-NAWND	French