unfastidious has no recorded synonym. 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. Its one recorded opposite is fastidious.
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.
not exacting in nutritional requirements
marked by an absence of due or proper care or attention to detail; not concerned with cleanliness
“unfastidious in her dress”
Synonyms, antonyms and the broader and narrower relations for unfastidious come from Princeton University’s WordNet. Full sources and licences.