What is foux di fa fa?

From "Flight Of The Conchords". Jemaine wants to impress the girls who work in the cafe by ordering a croissant. Bret tries to help him out and states that he's
very popular". Then Jemaine starts speaking in french and the sequence begins!

They're all singing foux di fa fa AYAH! over and over and saying random french words.

It's all mad fun!