Formal:
* Arrière-grand-mère: This is the most literal translation, using the French word for "great" (arrière) and "grandmother" (grand-mère).
Informal:
* Grand-mère-grand-mère: This is a more common and informal way to say "great grandmother" in Cajun French. It literally translates to "grandmother-grandmother."
* Grand-maman-grand-maman: Similar to the above, but using "grand-maman" which is another common term for grandmother in Cajun French.
* Mamou-grand: This is a more colloquial and regional term that might be used in some areas, meaning "great mother."
The best choice depends on the context and your personal preference.