Noun
grandmother-in-law (plural grandmothers-in-law)
A grandmother of one's spouse.
It turns out the date behind Meghan Markle's birthday, August 4th, also has a special meaning for her grandmother-in-law Queen Elizabeth. Source: Internet
This one is a gift from what we’ll call Emily’s grandmother-in-law once removed (I have no idea if there’s a word for what my mother-in-law is to Emily). Source: Internet