dash off has 5 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
write quickly
“She dashed off a note to her husband saying she would not be home for supper”
write down hastily
“She dashed off a letter to her lawyer”
Synonyms, antonyms and the broader and narrower relations for dash off come from Princeton University’s WordNet. Full sources and licences.