If the grandaunt is your father's sister:
* അമ്മയുടെ അനിയത്തി (ammayade aniyatthi): This means "mother's younger sister".
* അമ്മയുടെ ചേച്ചി (ammayade checchi): This means "mother's elder sister".
If the grandaunt is your mother's sister:
* അച്ഛന്റെ അനിയത്തി (achchande aniyatthi): This means "father's younger sister".
* അച്ഛന്റെ ചേച്ചി (achchande checchi): This means "father's elder sister".
Alternatively, you could use:
* പഴയ അമ്മായി (pazha ammayi): This literally means "old aunt" and could be used for any grandaunt.
It's important to note that the specific term used might vary depending on the speaker's dialect and region.