The best information I have found on "shortnin' bread" in the American 18th-19th c. sense is found in this online article, and in the "Dictionary of American Regional English."
"Shortnin' bread" 'was probably a simple quick bread (non-yeast) composed of flour (possibly cornmeal) and fat (probably lard." "...it is unlikely the recipe was for Scottish-type shortbread."

The article includes this reference- Dictionary of American Regional English, 2002, vol. 4, Joan Houston Hall, editor: etymological evidence strongly suggests shortening bread was "corn bread made with cooking fat, esp. lard," also known as crackling bread (p. 926).

In the American South, "'shortcake' or 'shortbread' also meant a combination of cornmeal and grease."

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