means The flat horizontal ledge at the bottom of a window, inside or out, where plants, cats, and forgotten coffee mugs tend to gather.

from A plain compound of two old English words: "window" itself comes from Old Norse "vindauga," literally "wind-eye" — the hole in the wall that let the wind look in. "Sill" descends from Old English "syll," a foundation beam or threshold, related to Old Norse "syll" and German "Schwelle," the doorsill you step over. Stitched together, the windowsill is quite literally the threshold of the wind-eye.