be (as) thick as thieves
US /bi əz θɪk əz θiːvz/
UK /bi əz θɪk əz θiːvz/

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.