Telling someone to stop behaving in an overbearing manner, or acting self-righteous or smugly superior, as if they know more, are better, or claim a higher moral ground than everyone else.
The first references to high horses were literal: ‘high’ or ‘great’ horses were very large. John Wyclif wrote of them in English Works, circa 1380, so the origin of the phrase goes back a long way.
In medieval England, a person’s rank was reflected by the size of the horse he rode. A noble or a person of importance would ride a large and expensive horse, one much taller and bigger than the horses ridden by commoners. The phrase “on one’s high horse” came to mean “superior.”
‘High’ has long been a synonym for ‘powerful’ or ‘not of the common people.’ This use of ‘high’ has also persisted in terms like ‘high and mighty,’ ‘high-handed,’ and ‘high finance,’ and in job titles like ‘high commissioner.’
Today, when we say that people are on their high horse we are implying a criticism of their haughtiness. The first riders of high horses didn’t see it that way. They were very ready to assume a proud and commanding position. Indeed that was the very reason they had mounted such a horse in the first place. And it’s also the reason for commissioning sculptures of themselves and their horses which presented both as larger than life.
The combination of the imagery of being high off the ground when mounted on a great war charger, looking down one’s nose at the common herd, and also being a holder of high office made it intuitive for the term ‘on one’s high horse’ to come to mean ‘superior and untouchable.’
By the 18th century, the use of such visual aids was diminishing and the expression ‘mounting one’s high horse’ changed from a literal to a figurative usage.
Deference to people in positions of power has diminished over the years and we tend nowadays to mock high and mighty people as being ‘on their high horse’ when they affect a superior and disdainful manner — the term is now rarely used for people who actually are powerful and remote.
And, since I’m neither powerful nor remote, applying the word ‘high’ to me probably just means I’ve had too much scotch.