From French 'dormir' (to sleep) — originally a window in a sleeping room; the name transferred from function to form.
A window set vertically in a structure projecting through a sloping roof, providing light and headroom to the space beneath the roof.
From French 'dormir' (to sleep), from Latin 'dormīre' (to sleep), from PIE *drem- (to sleep). A 'dormer window' was originally a 'dormant window' — a window in a sleeping room, specifically a window in the upper story where bedrooms were located. The name transferred from the room's function (sleeping) to the architectural feature (the projecting window structure) that made
The 'dormouse' — the small rodent famous for falling asleep at the Mad Hatter's tea party in 'Alice in Wonderland' — gets its name from the same root as 'dormer.' Both come from Latin 'dormire' (to sleep): the dormouse is literally 'the sleeping mouse,' and the dormer is 'the sleeping-room window.'