dun has 8 distinct synonyms spread across 2 of its 7 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.
persistently ask for overdue payment
“The grocer dunned his customers every day by telephone”
horse of a dull brownish grey color
a color or pigment varying around a light grey-brown color
“she wore dun”
1 of the 7 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for dun.
Synonyms, antonyms and the broader and narrower relations for dun come from Princeton University’s WordNet. Full sources and licences.