From Old English 'dox' (dark, swarthy) — shifted from an adjective for dark color to a noun for evening twilight.
The darker part of twilight, especially at the end of the day; the time just before nightfall when daylight is fading.
From Old English 'dox' or 'dosc,' meaning 'dark, swarthy,' which became Middle English 'dusk' as both an adjective and noun. The Old English adjective is of uncertain deeper origin, though it may be related to Proto-Germanic *duskaz and Latin fuscus ('dark, dusky'), both possibly from PIE *dʰeus- ('to become turbid, to become dark'). The word shifted from being primarily an adjective meaning 'dark-colored' to a noun