To be honest, when I first heard him say it I thought he was making it up. He has more than once tried to convince me something was USian or Texan that was merely Scottian. He likes to trick the dumb foreignor. But then I heard his nieces calling his mother “mee-maw”, so unless he briefed them ahead of time and they’re amazingly good actors, I’m ready to believe some Texans really call their grandmothers “mee-maw”.
Scott’s convinced that calling your grandmother “nana” is an Eastern European thing, but I know plenty of other Aussies with no Eastern European background who call their grandmothers “nana”.
So I’m driven to do some empirical research: Where are you from and what do (did) you call your grandmother? For extra credit: what do/did you call your grandfather? I called mine “papa”; Scott called his “grampa”.