IPA: /ˈɡrænˌɔnt/
KK: /ˈgrænˌɔnt/
A grandaunt is the sister of one's grandparent, or the aunt of one's parent.
My grandaunt shared many stories about our family's history.
Grandaunt is formed from "grand-" (meaning great or large, derived from Old French *grand*, meaning large or great) and "aunt" (from Old French *ante*, meaning father's or mother's sister). The word refers to the sister of one's grandparent, hence a great-aunt.
Think of 'grand' meaning great and 'aunt' as a family relation — a grandaunt is a great aunt, or the sister of your grandparent.