WordNet records no synonym and no antonym for scythe. 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.
an edge tool for cutting grass; has a long handle that must be held with both hands and a curved blade that moves parallel to the ground
cut with a scythe
“scythe grass or grain”
Synonyms, antonyms and the broader and narrower relations for scythe come from Princeton University’s WordNet. Full sources and licences.