What is the greek word for grand grandfather?

There isn't one single perfect equivalent in Greek for "grand grandfather" in the way we use it in English. Greek vocabulary tends to be more descriptive. You'd likely need a phrase, depending on the nuance you want to convey. Possible options include:

* προπάππους (propappos): This is probably the closest single word, literally meaning "great-grandfather." It's commonly used.

* παππούς του παππού (pappous tou pappou): This translates to "grandfather of the grandfather," which is more explicit.

The best choice depends on the context. `προπάππους` is generally sufficient, but if you need absolute clarity, the longer phrase might be better.