About ten years ago, there was a story about a British girl being found
guilty of causing death of child by shaking it. She was called an "au pair",
but definition or etymology was not given. I just found it:

au pair

NOUN:
A young foreigner who does domestic work for a family in exchange for room
and board and a chance to learn the family's language.
ETYMOLOGY:
French : au, at the + pair, equal.