from
The Collaborative International Dictionary of English v.0.48
Sill \Sill\ (s[i^]l), n. [OE. sille, sylle, AS. syl, syll; akin
to G. schwelle, OHG. swelli, Icel. syll, svill, Sw. syll,
Dan. syld, Goth. gasuljan to lay a foundation, to found.]
The basis or foundation of a thing; especially, a horizontal
piece, as a timber, which forms the lower member of a frame,
or supports a structure; as, the sills of a house, of a
bridge, of a loom, and the like. Hence:
(a) The timber or stone at the foot of a door; the threshold.
(b) The timber or stone on which a window frame stands; or,
the lowest piece in a window frame.
(c) The floor of a gallery or passage in a mine.
(d) A piece of timber across the bottom of a canal lock for
the gates to shut against.
[1913 Webster]
{Sill course} (Arch.), a horizontal course of stone, terra
cotta, or the like, built into a wall at the level of one
or more window sills, these sills often forming part of
it.
[1913 Webster]