on one's high horse

Idioms  

1. In an arrogant or condescending manner. For example, When they started talking about music, David got on his high horse and said that classical music was only fit for museums and archives. This expression, alluding to the use of tall horses by high-ranking persons, dates from the late 1700s. Similarly, off one's high horse means “less arrogantly, more humbly,” as in I wish she'd get off her high horse and be more friendly. It dates from the early 1900s, but is heard less often today.

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