slug has 13 distinct synonyms spread across 6 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. 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 unit of mass equal to the mass that accelerates at 1 foot/sec/sec when acted upon by a force of 1 pound; approximately 14.5939 kilograms
an amount of an alcoholic drink (usually liquor) that is poured or gulped
“he took a slug of hard liquor”
any of various terrestrial gastropods having an elongated slimy body and no external shell
Synonyms, antonyms and the broader and narrower relations for slug come from Princeton University’s WordNet. Full sources and licences.