granduncle-in-law Share definition Flag 1. The husband of one's grandparent's sister. 2. The brother of one's grandparent-in-law. My granduncle-in-law is a good person. granduncle-in-law by Elivaldeq November 24, 2019 0 0 Get merch Get the granduncle-in-law mug. Buy Now — $32.95 See more merch