Green’s Dictionary of Slang

1. (US) a countryman, esp. a New Englander; also attrib.

1840		R.H. Dana Two Years before the Mast (1992) 65: I’m F— T—, all the way from down-east. I’ve been through the mill, ground, and bolted, and come out a regular-built down-east johnny-cake.
1842		Dickens Amer. Notes (1985) 137: I’ma brown forester, I am. I an’ta Johnny Cake. There are no smooth skins where I live. We’re rough men there.
1931		E. Clark Innocence Abroad 180: Two of the antitheses in the web of them which formed Elinor Wylie were her love of elegance, and what she called her ‘johnny-cake side.’ The first she attributed to her Philadelphia mother [...] the second to her father, a product of ‘up-State’ Pennsylvania, with ancestral roots in New England.

2. (US/Can.) a French-born immigrant.

1983

Maledicta VII 24: French Canadians were called johnny cake, which Abraham Roback [...] says is from a phrase in a Montréal children’s doggerel, about 1900: ‘French peasoup and johnny cake / Make your father a bellyache’.