We had a thread about "au pair" over a year ago. The definition given at that time was that it meant an unpaid
babysitter.
I just now found in Burnside that the original usage was quite different:

"1897 Girl's Own Paper 16 Oct. "An arrangement..frequently made is for an English girl to enter a French, German or Swiss school and teach her own language in return for joining the usual classes. This is called being au pair."