Be on someone's high horse - Idioms by The Free Dictionary
be on (one's) high horse (redirected from be on someone's high horse)
be on (one's) high horse
To adopt an attitude of moral superiority. My sister was a troublemaker as a teenager, yet she's always on her high horse, lecturing me about everything. That radio host is known for always being on his high horse, despite the fact that everyone knows he's a creep.
be/get on your ˌhigh ˈhorse be annoyed because you think that somebody has not treated you with enough respect: When they suggested that she might have made a mistake, she got on her high horse and asked them how they dared question her ability.