means The horizontal piece forming the bottom of a window, door, or other framed opening — the ledge along the base.

from From Old English 'syll,' meaning a threshold, foundation beam, or base timber — the sturdy bottom member of a frame. It has Germanic cousins, related to Old Norse 'syll' and German 'Schwelle' (threshold), all pointing back to an ancient sense of a foundational sill or groundsel laid at the base of a structure. So a sill has always been a thing you build upon — or step over.