Why do we call them in-laws?
Maybe it has crossed your mind before that it’s because your spouse’s family members are related to you by law, not by blood and that’s why we say in-law but, that is not totally the case.
According to the Oxford English Dictionary, in-law refers to canon law which is a church’s set of rules and regulations that covers, among many other things, which relatives you’re prohibited from marrying.
Since the earliest known mention of the term in English is brother-in-law from the 14th century, it was most likely citing the canon law of the Catholic Church (as the Church of England wasn’t founded until the 16th century).
At its inception, in-law was specifically used to describe any non-blood relative that the church forbade you from marrying if your spouse died: your spouse’s siblings, parents, and children, and even your own stepsiblings, stepparents, and stepchildren.
So father-in-law, as The Word Detective explains, could’ve either meant your spouse’s father, or your mother’s new husband. But by the late 19th century—at which point the Church of England and other Protestant faiths had established their own canon laws with varying marriage rules—the colloquial definition had expanded to include all spousal relatives, and in-laws became a standalone phrase.
Source: Mental Floss
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