The Collaborative International Dictionary of English v.0.48:

Gammer \Gam"mer\ (g[a^]m"m[~e]r), n. [Possibly contr. fr.
   godmother; but prob. fr. grammer for grandmother. Cf.
   Gaffer.]
   An old wife; an old woman; -- correlative of gaffer, an old
   man.
   [1913 Webster]