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]