The noun window is from Middle English windoȝe, a borrowing from Old Norse vindauga, literally wind’s eye, from vindr, wind, and auga, eye.
The Scandinavian word replaced and finally superseded Old English éagþyrel, i.e. eyethirl, composed of the nouns eye and thirl.
The noun thirl denoted a hole, an aperture, and was derived from Old English þurh, thorough. It was long used in some dialects of English; for instance, the following is from The dialect of Craven, in the West-Riding of the County of York (1828), by William Carr (died 1843):
Thirl, the orifice of the nose; nose-thirl, alias nostril.
In standard Modern English therefore, thirl survives only in nostril, from Old English forms such as nosþirl.
The word eyethirl was also used with reference to the eye as the window of the body or soul. For example, a late-14th-century version of Ancrene Wisse (the Anchoresses’ Guide – between 1225 and 1240) contains, about Eve:
þe first þing þat brouȝth hire to synne was her eiȝe þirle (= the first thing that brought her to sin was her eyethirl).
In the late 13th century, English borrowed the Old French fenestre, which was in concurrent use with window until the mid-16th century. This French word was derived from Latin fenestra, itself related to:
– the Greek adjective φανερός (= phaneos), visible, manifest,
– the Greek verb φαίνω (= phaino), to bring to light, cause to appear, to come to light, appear, and to shine, give light.
In the Modern French fenêtre, the circumflex accent ^ is a trace of the etymological s present in English defenestration.