indecorous has 6 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. Its one recorded opposite is decorous.
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.
lacking propriety and good taste in manners and conduct
“indecorous behavior”
not in keeping with accepted standards of what is right or proper in polite society
“was buried with indecent haste”
Synonyms, antonyms and the broader and narrower relations for indecorous come from Princeton University’s WordNet. Full sources and licences.