E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Poor Jack or John (A).
Dried hake. We have john-dory, a jack (pike), a jack shark, and a jack of Dover, Probably the word Jack is a mere play on the word Hake, and John a substitute for Jack.
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Tis well thou art not fish; if thou hadst, thou hadst been poor-john.Shakespeare: Romeo and Juliet, i. 1.
We have a similar perversion in the school-boy proof that a pigeon-pie is a fish-pie. A pigeonpie is a pie-john, and a pie-john is a jack-pie, and a jack-pie is a fish-pie.