The term R.S.V.P. comes from the French expression "répondez s’il vous
plaît", meaning "please respond".
If R.S.V.P. is written on an
invitation it means the invited guest must tell the host whether or not they plan to attend the party.
It does not mean to respond only if you’re coming, and it does not mean respond only if you’re notcoming
(the expression "regrets only" is reserved for that instance).
It means
the host needs a definite head count for the planned event, and needs
it by the date specified on the invitation.
I think that my wife and I are at times the only one practicing this … argh …
Steve Holden's Blog 
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