Great-Aunt Heftythighs is a woman mentioned in Book 2, How to Be a Pirate.
Great-Aunt Heftythighs is brought up after the Hairy Hooligan Tribe opens the coffin of Grimbeard the Ghastly. A corpse is seen inside. Gobber comments there appears to be a resemblance to Stoick's family. Baggybum the Beerbelly responds, "I know what you mean ... There's a look of Great-Aunt Heftythighs." The corpse turns out to be Alvin the Poor-But-Honest-Farmer, who is very much alive.