Engaged in a vigorous campaign for a cause or belief, or fought in a religious war called a crusade.
From crusade, which comes from Old French croisade, from crois (cross). The term originally referred to the medieval religious wars, but by the 1700s it broadened to mean any passionate campaign or fight for a principle.
The word literally means 'marked with a cross'—crusaders wore red crosses on their clothes—and now we use it anytime someone fights hard for what they believe in, even if it has nothing to do with religion.
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