means A loose, lightweight garment worn in bed, designed for sleeping rather than seeing or being seen.

from A plain compound that does exactly what it says: "night" (Old English niht, the dark hours) plus "gown" (via Old French goune from Late Latin gunna, a loose robe or fur-lined garment). "Gown" arrived in English in the medieval period meaning a long, flowing outer garment; pair it with "night" and you simply specify which gown — the one for sleeping, not for court or church.