Why do I care? Well, IIUC the railway semaphore signals were named after this...

https://en.wikipedia.org/wiki/Railway_semaphore_signal

And those in turn (as Sipa told me) were inspiration for Dijkstra's concept, where V meant "vrijgeven" ("releasing" the railway section for the next train) or "verhogen" (increase). And for P "tentatieve passering" (trying to pass), he invented the word "prolagen" ("probeer verlagen": try to decrease). And passing the signal implies marking it as unsafe.

https://en.wikipedia.org/wiki/Semaphore_(programming)