Q: I wonder why English has only one term, “brother-in-law,” for three different kinds of relatives: your spouse’s brother, your sibling’s husband, and your spouse’s sibling’s husband.
A: You might also ask why something similar can be said of “sister-in-law,” which refers to three different relatives too.
This is probably because “brother-in-law” and “sister-in-law” originally referred not only to the various relatives involved but also to a prohibited relationship shared by them.
When “brother-in-law” entered English around 1300 and “sister-law” about 1440, the phrase “in-law” meant “in canon law,” as opposed to “in blood” or “by nature,” according to the Oxford English Dictionary.
The OED says the phrase was appended to names of relationship to indicate “the degrees of affinity within which marriage is prohibited; a brother-in-law or sister-in-law being, as regards intermarriage, treated ‘in law’ as a brother or sister.”
The word “affinity” here, Oxford says, refers to “relationship by marriage (as distinguished from relationship by blood).”
We won’t discuss the ins and outs of affinity in canon law, the ecclesiastical rules of the Catholic, Orthodox, and Anglican churches. Let’s just say that different churches have different rules for which relationships are impediments to marriage.
In an earlier post, we noted that the term “in-law” was once used in English to describe relationships that are now referred to with the term “step.” So, the expression “sister-in-law” once also meant stepsister.
Another old term, “inlaw,” used to mean the opposite of “outlaw” or, as the OED puts it, “one who is within the domain and protection of the law.”
Finally, in case you’re wondering, the use of the term “in-law” as a colloquial noun for any relative showed up first in the late 1800s. The earliest example in the OED is from the Jan. 24, 1894, issue of Blackwood’s Edinburgh Magazine:
“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.”
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