A ninny grannet is a slightly variated form of ninny (a simpleton or fool). While not as insulting as ninny, ninny grannet implies more fustration, sillyness and outright comic stupidity to the person its being directed.
John and I were playing Halo for X-box when he totally killed my ass with a rocket launcher. "Ninny Grannet!" I exclaimed with fustration, as I waited to respawn.