- an aunt of one's grandfather or grandmother.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
Examples from the Web for great-grandaunt
With languid step she ascended the stair, and was presently beside her great-grandaunt, Patricia Bower.Wilson's Tales of the Borders and of Scotland Volume 21
My great-grandaunt—I never saw her— Her ghost doesn't know me from another, She stalks up the attic stairs angrily.Goblins and Pagodas
John Gould Fletcher