Hint
Not to be confused with a certain programming language, nor with a term from knitting.
Solution
RSVP, for répondez s'il vous plaît (which literally means "respond if you please"—a polite way of saying "please respond"). To French speakers today, this sounds overly formal.
Congratulations to yesterday's solvers Anna J., Atticus, Jacob C., Jessica, Maddy, Peter M., Tori, Kim, Zachary S., Mr. Gregg, Graham, and Kate. Thanks to everybody who made a guess!
Ten years later, Dr. Shapiro revived Puzzle of the Day at Proof School, writing each day's puzzle on a name tag. After 600 puzzles or so, he was just starting to feel normal about students reading his chest all the time when campus closed and the puzzle, like the rest of our lives, moved online. New puzzles are posted daily on school days.
Want to catch up on old PotDs? There's an archive currently containing puzzles from March 2020 to March 2021.