silly has 15 distinct synonyms spread across 4 of its 5 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 seriousness; given to frivolity
“a dizzy blonde”
inspiring scornful pity
“how silly an ardent and unsuccessful wooer can be especially if he is getting on in years”
a word used for misbehaving children
“don't be a silly”
Synonyms, antonyms and the broader and narrower relations for silly come from Princeton University’s WordNet. Full sources and licences.