noun

• (architecture, also "window sill") A breast wall; window breast; horizontal brink which forms the base of a window.
  "She looked out the window resting her elbows on the window sill."
• (construction) A threshold; horizontal structural member of a building near ground level on a foundation or pilings, or lying on the ground, and bearing the upright portion of a frame; a sill plate.
• (geology) A stratum of rock, especially an intrusive layer of igneous rock lying parallel to surrounding strata.
  "Minor palingenetic magmas probably were generated at this time and intruded the mantling rocks in the form of small sills and apophyses […]."
• (anatomy) A raised area at the base of the nasal aperture in the skull.
  "the nasal sill"

noun

• (UK) A young herring.

adj

• (rare, slang) Silly.