Definition from Wiktionary, the free dictionary
co-brother-in-law (plural co-brothers-in-law)
- (uncommon) One's wife's sister's husband; or more generally one's spouse's brother-in-law, the brother of one spouse in relation to the siblings of the other spouse.
- (uncommon, in the plural) Men who marry sisters.
2008, Daniel Bornstein & David Peterson, editor, Florence and Beyond: Culture, Society and Politics in Renaissance Italy, page 350:
- Knowing that he would be susceptible to the arguments of his persuasive co-brother-in-law Antonio de' Medici, Saminiato tried to avoid his company.
- (uncommon) One's brother-in-law or sister-in-law's brother; that is, one's sibling's spouse's brother (one's sister's husband's brother or one's brother's wife's brother).
- (in the plural) Men whose brother and sister are married to each other.
- The term is generally used in translation in South India (Karnataka, Tamil Nadu, Andhra Pradesh) where the local language makes the distinction. In common speech in traditionally English-speaking countries, brother-in-law may be used.
- The primary usage is for one's wife's sister's husband.
- co-brother (Karnataka, Tamil Nadu)
- (secondary sense): co-uncle (in relation to a common niece or nephew)
Languages that have words for this relationship that are distinct from the word for brother-in-law. Not all are attested to have the second meaning above, though this may sometimes be an oversight.
one's spouse's brother-in-law