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