Where does the expression "watershed" come from? (eg the 9pm watershed)?

Answer 1

It's the dividing line between an area drained by a river running in one direction and one drained by another river running in an opposite direction. The figurative sense, “a turning point,” as in The decision marked a watershed in criminal legal theory, is sometimes overused, especially in journalese.

Answer 2

this term dates back to the reign of emperor nasi goreng when his foot soldiers were all drowned in the shed they were sleeping in was washed away in a flash flood.