This expression nowadays means "outside the bounds of acceptable behaviour", but its roots are much older, dating back more than seven centuries. A pale, or paling, was an upright stick [often pointed at the base] - and when these were all joined together, or stuck in the ground in a tight row they made a fence [also known as a pale] The word is from the same family as pole, impale and palisade.
"The Pale" was the fence that marked the boundary of someone's jurisdiction - most famously "The English Pale" across part of Ireland. The English considered that to travel 'beyond the pale' was to leave civilisation and the niceties of English society behind.
We have been working in the garden, and Bob put down an area of old bricks [reclaimed from the church building project] next to the shed. The wheelbarrow, incinerator and our new big brown garden wheelie bin stand there. But they were something of an eyesore. So he took some old wooden tongue&groove strips [also of no further use at UCF] and made a pale. Then he painted it with some preservative left over from another fence. Minimal cost, maximum recycling - and it all looks much better with the horticultural paraphernalia hidden behind a screen.
Here is Bob standing beyond the pale!!