means Happening during the course of a single night, or — more loosely — appearing suddenly as if it took no time at all.

from A plain compound of Old English roots: 'over' (ofer, meaning across or beyond) joined to 'night' (niht). Literally 'across the night,' it started as a humble adverb for the span of darkness between dusk and dawn — the time you'd stay somewhere or leave food to soak. The figurative 'overnight success,' meaning a transformation seemingly achieved in no time, is a much later extension, leaning on the idea that a single sleep is the shortest stretch a change could possibly take.