perhaps after John Abernethy †1831 English surgeon who treated maladies by diet
This expression was current at the end of John Abernethy's life, though journalism from the time suggests it was common knowledge that the doctor himself had nothing to do with the biscuits; cf. The Metropolitan, vol. 1, May-Aug, 1831, p. 182: “At this moment the public are deceived by supposing that a certain biscuit, abhorrent to our olfactory and gustatory senses, was the favorite breakfast and luncheon of Mr. Abernethy, whose name it bears, because the honest baker who invented it was called Abernethy, as many of our northern neighbors are. We venture to affirm, no such trash ever entered the worthy professor's stomach, and we know that what are called tops and bottoms were his choice, sometimes soaked in tea or eaten dry.” Also, John Epps, The Life of John Walker, M.D., London, 1831, p. 105: “From my history of the loaf of John Abernethy, some of my readers may observe on a label, ‘Abernethy biscuits,’ in a baker's window, in the Strand, which has lately met my eye. Le Boulanger n'est pas bete. [The baker is not stupid. ] The baker knows that Mr. Abernethy is, above all things, so attentive to the digestive organs of his patients, that, by association of his name with the biscuits, the valetudinarians must flock to him for his biscuits, like doves to the windows.” It seems likely, though conjectural, that some baker did name the biscuit after the doctor.