Au fait – this is an example of a French expression that has become part of the English Language. It means to have good detailed knowledge of something. (This is not slang but a very British English expression.)

Au fait是英语从法语中借用的表达之一。它的意思是对某事物有非常细致的了解。（它不是俚语却是非常地道的英式表达）

例句：

She is au fait with the company's rules and regulations.

她对公司的规章制度了然于心。