The Collaborative International Dictionary of English v.0.48:

Sill \Sill\, n. [Cf. Thill.]
   The shaft or thill of a carriage. [Prov. Eng.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sill \Sill\, n. [Cf. 4th Sile.]
   A young herring. [Eng.]
   [1913 Webster]

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]

WordNet (r) 3.0 (2006):

sill
    n 1: structural member consisting of a continuous horizontal
         timber forming the lowest member of a framework or
         supporting structure
    2: (geology) a flat (usually horizontal) mass of igneous rock
       between two layers of older sedimentary rock

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "sill":
   base, basement, basis, bearing wall, bed, bedding, bedrock,
   doorsill, doorstone, floor, flooring, fond, footing, foundation,
   fundament, fundamental, ground, grounds, groundsel, groundwork,
   hardpan, mudsill, pavement, principle, radical, riprap,
   rock bottom, rudiment, seat, solid ground, solid rock, stereobate,
   stylobate, substratum, substruction, substructure, terra firma,
   threshold, underbuilding, undercarriage, undergirding,
   underpinning, understruction, understructure, window sill