WordNet records no synonym and no antonym for sill. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure
(geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock
Synonyms, antonyms and the broader and narrower relations for sill come from Princeton University’s WordNet. Full sources and licences.