“Taps” originally began as a signal to extinguish lights. Up until the Civil War, the infantry call for “Extinguish Lights” was the one set down in the Infantry manuals which had been borrowed from the French.

No, irony for a folkie website, the French original was the signal to man the piquet lines. Tattoo is the beginning of inspection of the Quarter(s) Guard. Taps is the end of inspection and the beginning of the night watch; head counts &c*.

Here's that word again: céleustique - “(Militaire) Qui a un rapport avec l’art de transmettre des ordres par des signaux sonores.”

*It's also the start of light discipline and so was subject to “Daylight Savings Time,” typ. 10:00PM summer; 9:00PM winter.