表達 「be as thick as thieves」 的意思和偷東西並無關連，而是用來形容友人關係密切、親密無間，彼此間沒有隔閡或秘密。這個表達通常用來暗示兩人或多人就像 「thieves（小偷）」 聚在一起做壞事一樣 「thick（密集地）」，悄悄地盤算或談論不可告人的事情。
在使用時，可以去掉表達中的第一個 「as」，即 「be thick as thieves」，意思相同。
My daughter and her friend are thick as thieves and spend all their free time together. We often have no idea what they are up to, but they really enjoy themselves.
The kids are as thick as thieves today. Whilst I was just on the phone they painted their faces with lipstick. They just looked at me as if nothing had happened!
Jack and Jim were as thick as thieves when they were younger, but when they went to university in different towns it just wasn’t the same.