What Is The Origin Of The Phrase 'Once Bitten, Twice Shy'?

It seems likely that this something similar to this phrase was first recorded as the moral of one of Aesop's fables from the 6th century BC, although as a proverb it could be even older. At any rate, the 15th century printer William Caxton retells one Aesop's fable with the following phrase: "He that hath been once beguiled by some other ought to keep him well from the same," which has roughly the same meaning.

The phrase is also used, almost as we use it today, by the 18th century English author Surtees.

It basically means that one bad experience will teach you to be more cautious in future. The German version of the saying is "Once burned, twice shy" which is almost the same. ("Shy" here means afraid, as a horse "shies" away from danger, not our more modern meaning of being nervous in front of strangers.)
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The Internet consensus is that the phrase traces back to an English printer (William Caxton), the first to publish a translation in to English of Aesop's tales -- in 1484.

Aesop was a Greek writer who lived about the 6th century BC.  There are many myths and fuzzy info about his actual life.  By tradition he was supposed to have been a slave, and his tales were not so much made up by him but transcriptions of folk fables that were widespread in his culture.

The moral of one of Aesop's stories was translated by Caxton as "He that hath ben ones begyled by somme other ought to kepe hym wel fro(m) the same".  That has been rephrased and rephrased over the years, until early into the 20th century when it evolved into the tidy current phrasing.

The fable itself is about a wolf who threatens to eat a dog.  The dog says "Not now, wait until after I have been fed and am fatter".  Dumb wolf lets dog go.  When wolf returns he can't get at the dog to eat the dog, and the dog says something along the lines of "Don't be so stupid again".  Modern Moral: "Once bitten twice shy", although Caxton's "He that has been fooled once by another ought to keep away from that same person in the future" makes more sense in context of the original story.