Used to call one of my grandmothers Nenah, but I don't know where that came from.
I would hazard a guess that this is a form of Nanna - a very common Englisj form for Grandmother.

In Northam[tonshire, the usual form was Nanna and Pap or Pappa.

(One's grandson was commonly referred to as, "Moi Lil' ole bwoy.") ... and there aren't any typos there - that's the way they said it!