loose has 24 distinct synonyms spread across 14 of its 18 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 2 recorded opposites are compact, tight.
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 compact or dense in structure or arrangement
“loose gravel”
not tight; not closely constrained or constricted or constricting
“loose clothing”
having escaped, especially from confinement
“a convict still at large”
casual and unrestrained in sexual behavior
“her easy virtue”
2 of the 18 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for loose.
Synonyms, antonyms and the broader and narrower relations for loose come from Princeton University’s WordNet. Full sources and licences.