thick as thieves
非常亲密或心知肚明
口语·形容非常亲密的关系
英文释义
very close and intimate, often in a mischievous or secretive way
例句
Those two have been thick as thieves since kindergarten.
典故来源
源自18世纪英国俚语，指盗贼间紧密合作，后泛化形容朋友间极其亲密。