The story
From co- + brother-in-law or co-brother + -in-law.
Etymology from Wiktionary, CC BY-SA 4.0.
co-brother-in-law
Today it means: 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. Full definition →