thick as thieves --------- 亲密无间的（非正式）

英文释义
(INFORMAL) Adjective phrase describing a group of people who are closely and often conspiratorially connected.

例句
My two gossipy neighbors are thick as thieves and I will never tell any secrets to either of them because the other will hear about it immediately.
我有两个亲密无间的多嘴邻居，我不会跟他们中的一个讲任何秘密，因为另外一个人会马上知道。