Etymology
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brother-in-law (n.)

"brother of one's husband or wife," also "brother of one's sister's husband," c. 1300; also brother in law; see brother + in-law. In Arabic, Urdu, Swahili, etc., brother-in-law, when addressed to a male who is not a brother-in-law, is an extreme insult, with implications of "I slept with your sister."

updated on October 10, 2017

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Definitions of brother-in-law from WordNet

brother-in-law (n.)
a brother by marriage;
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.