be (as) thick as thieves

US /bi əz θɪk əz θiːvz/
UK /bi əz θɪk əz θiːvz/
"be (as) thick as thieves" picture
1.

to be very close friends and share secrets, often in a way that excludes others

:
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Those two have been as thick as thieves since childhood.
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The new neighbors are already as thick as thieves with the family next door.
:
thickthick and fastthick on the groundthrough thick and thin(as) thick as two short planksfaults are thick where love is thin