Don't know the origin, but with no technical background past using Linux, I only ever heard of brownouts in contexts of failing (often 3rd world) electrical infrastructure. Mostly Africa and South America (don't mean to offend anybody living there, I know they're vast continents with many rich/infrastructure-stable countries too).

Origin is the electrical grid overload which caused incandescent lights to literally "brown out", as has been mentioned here.

Later is was coopted to mean any problems with power supply not including outright drop to zero-zero/disconnections. cf microcontroller brown-out handling, also mentioned above.

Then later it seems it was generalized to mean sort-of-non-terminal problem with supply of most anything.

Very odd to call it a 3rd world phenomenon when California calls it a normal day.