co-uncle

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.

Japanese Meaning

婚姻などの関係で、実際の血縁による兄弟関係がないにもかかわらず、甥や姪に対して共通の叔父として認められる男性（義叔父）を指す。 / 二人の男性が、それぞれ結婚している兄弟姉妹の子供に対して、共通の叔父としての関係を持つ場合に使われる表現。

What is this buttons?

Although he isn't my brother, my co-uncle always supports our niece and nephew during family gatherings.

Google Translation / DeepL Translation

彼は私の兄弟ではないが、義理の叔父は家族の集まりでいつも姪や甥を支えてくれる。

Details

Sense（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.

Cicero continued however to write anxiously to Atticus, his co-uncle, about the young man's remarkable gifts and unsatisfactory character.

[Cicero's younger brother married Atticus's sister, and the couple had a son Quintus; Cicero and Atticus were thus co-uncles to Quintus, the nephew they had in common]

Google Translation / DeepL Translation