Outside the bounds of morality, acceptable behavior or good judgement, etc.

Example

"He was so rude at that party. His behavior was beyond the pale"

Etymology

From pale (“jurisdiction of an authority, territory under an authority's jurisdiction”), suggesting that anything outside the authority's jurisdiction was uncivilized. The phrase was in use by the mid-17th century, and may be a reference to the general sense of boundary, but is often understood to refer specifically to the English Pale in Ireland. In the nominally English territory of Ireland, only the Pale fell genuinely under the authority of English law, hence the terms within the pale and beyond the pale. 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.

Sources

Wiktionary