Outside the bounds of morality, the law, acceptable behaviour, and good judgement, etc.
Pale, as a noun, is an old term for a pointed piece of wood driven into the ground, usually a fencepost or a fence, thus a barrier. It has also been used to mean “territory under an authority’s jurisdiction.” The word is from the same Latin source as pole, impale, paling, and palisade.
The phrase “beyond the pale” dates back to 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.
In particular, the term was used to describe various defended enclosures of territory inside other countries. For example, the English pale in France in the fourteenth century was the territory of Calais, the last English possession in that country. The best-known example is the Russian Pale which, between 1791 and the Revolution of 1917, were specified provinces and districts within which Russian Jews were required to live.
Another famous one is the Pale in Ireland, the part of the country which England directly controlled — it varied from time to time, but was an area of several counties centred on Dublin. The first mention of the Irish Pale is in a document of 1446–7. Though there was an attempt later in the century to enclose the Pale by a bank and ditch (which was never completed), there never was a literal fence around it.
To travel outside of that boundary, beyond the pale, was to leave behind all the rules and institutions of English society, which the English modestly considered synonymous with civilization itself.
A much later example is, “By its conversion England was first brought, not only within the pale of the Christian Church, but within the pale of the general political society of Europe.”
(The History of the Norman Conquest, by Ernest A Freeman, 1867.)
The boundary of the Ashdown Forest (a royal hunting forest) was also known as the Pale, consisting of a paled fence and a ditch inside, to allow deer to jump in, but not back out.
To say that something is “beyond the pale” is often merely a matter of opinion. One person’s “beyond” may be another person’s “within.” My mother and I often used to have such differences of opinion!