From World Wide Words:

Pale is an old name for a pointed piece of wood driven into the ground and — by an obvious extension — to a barrier made of such stakes, a palisade or fence. Pole is from the same source, as are impale, paling and palisade. This meaning has been around in English since the fourteenth century and by the end of that century pale had taken on various figurative senses — a defence, a safeguard, a barrier, an enclosure, or a limit beyond which it was not permissible to go. The idea of an enclosed area still exists in some English dialects.
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The earliest figurative sense that’s linked to the idiom was of a sphere of activity or interest, a branch of study or a body of knowledge, which comes from the same idea of an enclosed or contained area; we use field in much the same way. This turned up first in 1483 in one of the earliest printed books in English, The Golden Legende, a translation by William Caxton of a French work.

The Phrase Finder adds that the first printed reference of the phrase “beyond the pale” (rather than just the word pale in its figurative sense) comes “from 1657 in John Harington’s lyric poem The History of Polindor and Flostella.”

In that work, the character Ortheris withdraws with his beloved to a country lodge for ‘quiet, calm and ease’, but later venture further – ‘Both Dove-like roved forth beyond the pale to planted Myrtle-walk’. Such recklessness rarely meets with a good end in 17th century verse and before long they are attacked by armed men with ‘many a dire killing thrust’. The message is clearly, ‘if there is a pale, you should stay inside it’, which conveys exactly the meaning of the phrase as it is used today.