To be on one's high horse
To ride the high horse
Get off your high horse
To be stuck up and arrogant. To look down on other people. To have an attitude of arrogant superiority.
This phrase originated from Medieval England, when knights and royalty rode 'great horses', large horses bred to be riden into battle by a heavily armoured knight. It came to be that anyone ridding on a high horse was an important person, or at least considered himself to be one. Hence it came to mean one who thinks e is better than the masses.