What is the origin of the term 'Au pair'

Yes it is from French:

Au pair:

late 19th century: from French, literally ‘on equal terms’. The phrase was originally adjectival, describing an arrangement between two parties paid for by the exchange of mutual services; the noun usage dates from the 1960s.

The expression was originally used to indicate economic parity for instance:

The title comes from the French term au pair, meaning "at par" or "equal to", indicating that the relationship is intended to be one of equals: the au pair is intended to become a member of the family, albeit a temporary one, rather than a traditional domestic worker.

Although it is not as simple to acquire au pair status as it was in 1840, the expression was used to indicate economic parity between the "employer" and their "employee". The expression au pair was used in this sense in 1840 by Honoré de Balzac:

• Sylvie Rogron fut envoyée à cent écus de pension en apprentissage....Deux ans après, elle était au pair: si elle ne gagnait rien, ses parents ne payaient plus rien pour son logis et sa nourriture (BALZAC, Pierrette, 1840, p.17).

(Wikipedia)