Au pair is the French term for a live-in nanny.
Spot on!
In the UK the term "au pair" usually refers to a female student who comes to the UK to study English, lives with a British family, and does housework in return for living expenses. Au pairs are a perennial subject of male fantasy.
Here are a couple of lines from a song called, appropriately,
Fantasy Girls, from an album of the same name by British group Charlie (1976).
Each night I sit in sleazy flea pits to watch the rude bits
The sexy au pair with the black leather hair and the pendulous pair.
The original cover of this album was replaced by more mundane artwork for release in the US. Maybe it had something to do with image copyright.