brother-in-law

American  

[bruhth-er-in-law] / ˈbrʌð ər ɪnˌlɔ /

noun

plural

brothers-in-law

1. the brother of one's spouse.

2. the husband of one's sister or brother.

3. the husband of one's spouse's sister or brother.

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brother-in-law British  

noun

1. the brother of one's wife or husband

2. the husband of one's sister

3. the husband of the sister of one's husband or wife

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Etymology

Origin of brother-in-law

First recorded in 1350–1400; Middle English brother in law(e); see origin at brother, in, law ¹; from Middle English in-lawe “in law,” i.e., “a person within the regulation and protection of the law,” based on the prohibition by Roman civil law and, later, Christian canon law, of marriages within four degrees of consanguinity, i.e., up to and including first cousins

Example Sentences

While we were talking, her brother-in-law, Jeremy, came outside.

From Salon • Apr. 19, 2026

Ahmad said that those were photos from a trip to see his brother-in-law, who was training at a U.S. military base.

From Slate • Apr. 19, 2026

Crew members tried to give him first aid, but to no avail, said Alok Singh, the late captain’s brother-in-law.

From The Wall Street Journal • Apr. 5, 2026

"I'd always wanted to do a practical job, and my brother-in-law had just gone through the training for a train driver and said, 'why don't you try it?'."

From BBC • Mar. 21, 2026

After Lizzie died in July 1921, Mollie’s brother-in-law, Bill Smith, had presented his suspicions to authorities that she’d been slowly poisoned, but by August they had still not looked into the case.

From "Killers of the Flower Moon" by David Grann

Related Words

• aunt
• cousin
• father
• folk
• mother
• niece
• sibling
• uncle