of a man, an uncle of one's niece or nephew who is not one's brother; a co-brother-in-law as common uncle to the children of two men's married siblings.
a man's brother, as uncle in common to the children of a third siblingMy nephew came up for the summer with my brother and co-uncle George.
Synonyms
co-brother-in-law (relative to the married siblings rather than to the children; however, this term generally refers to a different relationship, that of two men who marry sisters)