on one's high horse, to be

high horse

An attitude of moral superiority. Typically used in the phrases "get off (one's) high horse" and "on (one's) high horse." My sister needs to get off her high horse and stop lecturing me because I know she was a troublemaker as a teenager! The radio host is known for always being on his high horse, despite the fact that everyone knows he's a creep.
See also: high, horse

on one's high horse, to be

To put on airs; to behave arrogantly. As long ago as the fourteenth century, persons of high rank rode very tall horses, a custom that came to symbolize superiority and arrogance. By 1800 or so, to be or to get on one’s high horse meant to act superior, with or without justification.
See also: high, on