“RSVP” is an initialism of the French phrase répondez s’il vous plaît, meaning “please respond.” It was adopted by the English in the late 18th Century, during a brief fad for copying French mannerisms and etiquette. It was codified in the 20th century by etiquette writer Emily Post, who held that failing to respond to an invitation containing the phrase “RSVP” was the worst thing a person could do, worse than murder or treason. But not, presumably, worse than copying the French.
