idle has 15 distinct synonyms spread across 8 of its 10 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 3 recorded opposites are busy, run, work.
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.
without a basis in reason or fact
“baseless gossip”
the state of an engine or other mechanism that is idling
“the car engine was running at idle”
Synonyms, antonyms and the broader and narrower relations for idle come from Princeton University’s WordNet. Full sources and licences.