joker has 2 distinct synonyms spread across 2 of its 4 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.
a person who does something thoughtless or annoying
“some joker is blocking the driveway”
an inconspicuous clause in a document or bill that affects its meaning in a way that is not immediately apparent
“when I demanded my money he showed me the joker in the contract”
a playing card that is usually printed with a picture of a jester
Synonyms, antonyms and the broader and narrower relations for joker come from Princeton University’s WordNet. Full sources and licences.