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

1.
sangat rapat, seperti isi dengan kuku
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.
Dua orang itu sudah sangat rapat sejak kecil.
•
The new neighbors are already as thick as thieves with the family next door.
Jiran baru itu sudah sangat rapat dengan keluarga sebelah.