undesirable has 2 distinct synonyms spread across 2 of its 3 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. Its one recorded opposite is desirable.
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 worthy of being chosen (especially as a spouse)
one whose presence is undesirable
“rounding up vagrants and drunks and other undesirables”
Synonyms, antonyms and the broader and narrower relations for undesirable come from Princeton University’s WordNet. Full sources and licences.