sneak has 29 distinct synonyms spread across 6 of its 8 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.
put, bring, or take in a secretive or furtive manner
“sneak a look”
a person who is regarded as underhanded and furtive and contemptible
marked by quiet and caution and secrecy; taking pains to avoid being observed
“a furtive manner”
Synonyms, antonyms and the broader and narrower relations for sneak come from Princeton University’s WordNet. Full sources and licences.
someone acting as an informer or decoy for the police