be (as) thick as thieves
US /bi əz θɪk əz θiːvz/
UK /bi əz θɪk əz θiːvz/
1.
大の仲良し, 非常に親しい
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.
あの二人は子供の頃から大の仲良しだ。
•
The new neighbors are already as thick as thieves with the family next door.
新しい隣人たちは、すでに隣の家族と大の仲良しだ。