The word partner is an alteration of partner ‘partner, joint heir’, from Anglo-Norman French parcener, based on the Latin for partition, and thus related to the group of words at part. The change from ‘c’ to ‘t’ was to match part. Partner for a spouse dates from the early 17th century; the term came to be commonly applied to each person of an unmarried couple sharing a home towards the end of the 20th century. Partner in crime dates from the early 19th century.