Buzz
While we’re busy remembering that the plural form of mother-in-law is mothers-in-law, not mother-in-laws, we often overlook a more intriguing question: Why do we even call them in-laws?
You might think it's because your spouse's relatives are connected to you legally, rather than by blood—but the law in question isn’t tied to the marriage certificate your officiant sends to the county clerk. According to the Oxford English Dictionary, in-law refers to canon law, a church’s collection of rules that dictate, among other things, which relatives you're forbidden to marry. Since the earliest known use of the term in English is brother-in-law from the 14th century, it likely refers to the canon law of the Catholic Church (since the Church of England wasn’t established until the 16th century).
When the term in-law was first used, it described any non-blood relative whom the church prohibited you from marrying if your spouse passed away: your spouse’s siblings, parents, children, and even your own stepfamily—stepsiblings, stepparents, and stepchildren. As The Word Detective explains, father-in-law could have referred either to your spouse's father or to your mother’s new husband. By the late 19th century, when the Church of England and other Protestant churches had set their own canon laws with differing marriage rules, the term had broadened to encompass all relatives by marriage, and in-laws became its own distinct term. The earliest known reference comes from a 1894 article in Blackwood’s Edinburgh Magazine, which states that “the position of the ‘in-laws’ (a happy phrase which is attributed with we know not what reason to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness.”
In other words, the strain between individuals and their in-laws has existed as long as the term itself. If this is part of the reason you're here, take a look at these 12 tips from the 19th century on how to manage such relationships.
