Noun
(uncommon) One's spouse's brother-in-law (one's spouse's sibling's husband), especially one's wife's sister's husband; either of two (or more) men who marry sisters, in relation to the other; the brother of one spouse in relation to the siblings of the other spouse.
(uncommon) One's brother-in-law's or sister-in-law's brother; that is, one's sibling's spouse's brother; either of two (or more) men whose siblings are married to each other.
Source: en.wiktionary.org