Green’s Dictionary of Slang

John Dory n.1

[the rhyme is convenient, rhy. sl. does not exist pre-1800. The name, orig. given to a fish (aka St Peter's Fish), is that used of an English seaman, possibly a pirate, who, as claimed in a popular ballad (printed 1609), traitorously promised the then king of France to bargain for his own freedom by handing over a number of English prisoners but was ultimately captured by an English ship; all available cites suggest a dull repetitousness in his pleading]

a dull, repetitous speaker or story-teller.

Show quotations
[UK]T. Duffet Epilogue Spoken by Heccate and Three Witches 42: Have we not seen, O whorson Rogue John Dory [...] Catch’d ten times o’re with old new dress’d Story].
[UK]Rochester ‘The Young Statesman’ in Works of Rochester (1721) 53: These will appear such Chits in Story, / ’Twill turn all Politicks to Jests / To be repeated like John Dory, / When Fiddlers sing at Feasts].
J. Phillips [trans.] ‘Something instead of an Epistle to the Reader’ in Cervantes Don Quixote n.p.: While our damn’d Hum-drum Dottrels, dull John Dory's, / But rather digg'd our Graves then wrought our Story’s.