An acronym for the French phrase 'Répondez s'il vous plaît' meaning 'please respond'—a request on invitations for the recipient to confirm they will attend.
From French 'Répondez' (respond) 's'il' (if it) 'vous' (you) 'plaît' (pleases). This French convention became standard in English social etiquette in the 1800s and remains used today.
RSVP has basically defeated itself—most people ignore it now, so event planners can never predict attendance. It's hilarious that this formal French politeness rule is now widely ignored, making the acronym basically useless!
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