ride the high horse 趾高氣昂
Get off your high horse.
In medieval times,people of noble birth rode the best horses,which were generally bigger,stronger,and taller.To ride a high horse was to be in a higher position,both physically and economically,and it afforded the rider the ability to talk down to less privileged people.We now use the phrase to refer to any instance in which someone affects an air of moral superiority.