*Quizmonster, at the answer bank.co.uk: “In olden times, there were lots of proverbial-type phrases involving eggs. People were said to be ‘as like as two eggs’ (Shakespeare 1611) or were advised not to ‘put all their eggs in one paniard’ (1666). The earliest-recorded appearance of ‘as sure as eggs are eggs’ was in a dictionary of slang published in 1699. The ‘eggs is eggs’ version is just a more modern variation. It’s pretty clear that the basic idea is at least half a millennium old, in other words, if it was published as slang in the 1600s.”
From the website of the British Library:
“William Caxton, (born c. 1422, Kent, England—died 1491, London), the first English printer, who, as a translator and publisher, exerted an important influence on English literature.
In 1438 he was apprenticed to Robert Large, a rich mercer, who in the following year became lord mayor of London. Large died in 1441, and Caxton moved to Brugge, the centre of the European wool trade; during the next 30 years he became an increasingly prosperous and influential member of the English trading community in Flanders and Holland. In 1463 he took up duties as “Governor of the English Nation of Merchant Adventurers” in the Low Countries—a post of real authority over his fellow merchants. Sometime in 1470 he ceased to be governor and entered the service of Margaret, duchess of Burgundy, possibly as her financial adviser.
In that period Caxton’s interests were turning to literature. In March 1469 he had begun to translate Raoul Le Fèvre’s Recueil des histoires de Troye, which he laid aside and did not finish until September 19, 1471. In Cologne, where he lived from 1470 to the end of 1472, he learned printing. In the epilogue of Book III of the completed translation, entitled The Recuyell of the Historyes of Troye, he tells how his “pen became worn, his hand weary, his eye dimmed” with copying the book; so he “practised and learnt” at great personal cost how to print it. He set up a press in Brugge about 1474, and the Recuyell, the first book printed in English, was published there in 1475. Caxton’s translation from the French of The Game and Playe of the Chesse (in which chess is treated as an allegory of life) was published in 1476. Caxton printed two or three other works in Brugge in French, but toward the end of 1476 he returned to England and established his press at Westminster. From then on he devoted himself to writing and printing. The first dated book printed in English, Dictes and Sayenges of the Phylosophers, appeared on November 18, 1477.
Because of the diversity of English regional dialects at this time, and the changing nature of the language, it was difficult for Caxton to choose which words to use in his translations. In this book, Caxton tells the story of some merchants from the North of England trying to buy eggs from a woman in the South of England. The northerner uses the word egges, derived from Old Norse, but the Southern woman, who uses the word eyren from the Old English, does not understand. A humorous misunderstanding ensues.
Caxton needed to write in a dialect that would be understood by as many readers as possible, and decided to base his translations on a London dialect aimed at ‘a clerke and a noble gentylman’. Caxton felt his own Kentish dialect was ‘broad and rude’; the London dialect was thought to be more refined, and included many words derived from French or Latin.”