The story
From the (probably unserious) suggestion that witches could not fly through them.
Etymology from Wiktionary, CC BY-SA 4.0.
witch window
Today it means: A window placed diagonally, with its long edge parallel to the roof, in the gable-end wall of a house (chiefly in Vermont in the United States), where a window oriented vertically or horizontally would not fit. Full definition →