co-uncle

Noun

 * 1) 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.
 * 2) a man's brother, as uncle in common to the children of a third sibling
 * My nephew came up for the summer with my brother and co-uncle George.
 * My nephew came up for the summer with my brother and co-uncle George.

Synonyms

 * (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)