c. 1200, literally "wind-eye," from Old Norse vindauga, from vindr "wind" (see wind (n.1)) + auga "eye" (from PIE root *okw- "to see"). Replaced Old English eagþyrl, literally "eye-hole," and eagduru, literally "eye-door." Compare Old Frisian andern "window," literally "breath-door."
Originally an unglazed hole in a roof. Most Germanic languages later adopted a version of Latin fenestra to describe the glass version (such as German Fenster, Swedish fönster), and English used fenester as a parallel word till mid-16c.