The English word "window" is one of the most quietly poetic of the Norse borrowings into our language. It comes from Old Norse vindauga, a compound of vindr ("wind") and auga ("eye"). A window was, in plain Norse, a "wind-eye" — an opening in a wall through which the wind could pass and through which the wall could see out.
A Pre-Glass World
The metaphor makes more sense when you remember what a window was, before glass. In a Viking longhouse, a window was simply a hole near the roof, often closed with a wooden shutter or a piece of stretched animal skin or oiled cloth. There was no pretence that a window kept the wind out — it let smoke escape from the central fire and let in a little weak northern light. The "wind-eye" was a working description.
Replacing an Older English Word
Old English already had its own perfectly good word for a window opening: ēag-þyrel, "eye-hole" or "eye-thirl" (where thirl means a hole, the same root that gives us nostril, "nose-thirl"). The Norse vindauga arrived with the Vikings and slowly displaced it. By the late Middle Ages, window had won; eag-thirl faded out of use entirely.
Other Norse Loans Inside the House
So many household words in modern English are gifts of the Vikings that you can build a sentence almost entirely from them: They took their knives and skirts and sky-blue eggs to the window. Take, knife, skirt, sky, egg, they, their — all from Old Norse. The Vikings did not just settle the land; they reshaped the everyday vocabulary of the people who lived on it.