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

1.
çok yakın arkadaş, sırdaş
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.
O ikisi çocukluktan beri çok yakın arkadaş.
•
The new neighbors are already as thick as thieves with the family next door.
Yeni komşular, yan komşu aileyle şimdiden çok yakın arkadaş oldular.