A brother-in-law is your partner's brother, or your sister's husband. My brother-in-law.
As with the sister, the one word covers two quite different relationships. A brother-in-law is chosen by somebody else, which is the whole comedy of the word.
1nounbrother-in-lawYour partner's brother, or your sister's husband.
“His brother-in-law came for a fortnight and stayed three years.”
brother-in-law is a relative.
Middle English brother-in-lawe; the law is church law about marriage.
Middle English
brother-in-lawe
English, today
brother-in-law
From Middle English brother-in-lawe; equivalent to brother + -in-law.
Etymology from Wiktionary, CC BY-SA