mother-in-law

The term "mother-in-law" is a compound native to English, formed from its constituent words "mother," "in," and "law." The use of "in-law" to denote a relationship created by marriage, rather than by blood, dates to the 14th century. The "law" in this context refers to the canonical law of the church, which governed and defined the degrees of affinity and the prohibitions on marriage within certain family connections. Thus, a "mother-in-law" is literally the mother one gains through the legal and religious contract of marriage, distinguishing her from one's biological mother.

This is a compound noun formed from three English words: "mother" (from Old English mōdor, meaning female parent), the preposition "in" (from Old English in), and "law" (from Old English lagu, meaning rule or ordinance, but here used in the specific sense of relationship by marriage).