"Get off your high horse" is an idiom people
retort with in an argument when the other person is acting like a
holier-than-thou being when in reality, they aren't. To put it in a
simpler manner, to tell people to stop acting as if they're the top authority when they aren't.
Person A: "I am completely
entitled to go around dismissing people when they
do something completely normal just because they're wrong and I'm
ultimately right."
Person B: "Get off your high horse."