be (as) thick as thieves

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

amigos inseparáveis, muito próximos

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

:
Those two have been as thick as thieves since childhood.
Aqueles dois são amigos inseparáveis desde a infância.
The new neighbors are already as thick as thieves with the family next door.
Os novos vizinhos já são amigos inseparáveis da família ao lado.