c.1200, dosk "obscure, to become dark," perhaps from Old English dox "dark-haired, dark from the absence of light" (cognate with Swedish duska "be misty," Latin fuscus "dark," Sanskrit dhusarah "dust-colored;" also compare Old English dosan "chestnut-brown," Old High German tusin "pale yellow") with transposition of -k- and -s-, perhaps via a Northumbrian variant (compare colloquial ax for ask). But OED notes that "few of our words in -sk are of OE origin." A color word originally; the sense of "twilight" is recorded from 1620s.