A list of words I have heard used to refer to one's father or mother's mother and father.

Grandmother.

Nana.

Grandma.  

Nunna. 

Meemaw (what the fuck, Kentucky?)

Gran.

Nan.

Granny.

G-ma.

Grandpa.

Grandfather.  

Grandad.

Pep-pep (hahahahahaha)

Pop up.

Gramps.

Grandaddy.

What else?  This stuff's funny to me.