Once Bitten, Twice Shy

The underlying idea is genuinely ancient, traceable to Aesop's fables via William Caxton's 1484 English translation: "He that hath ben ones begyled by somme other ought to kepe hym wel fro the same." But the exact, catchy modern wording, "once bitten, twice shy," took a lot longer to settle into its final form, only clearly documented in English from the late 19th century, appearing in phrase collections and picked up by novelists including Joseph Conrad and Aldous Huxley. The core wisdom is centuries older than the precise turn of phrase that carries it today.