riffle has 11 distinct synonyms spread across 4 of its 6 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.
shuffle (playing cards) by separating the deck into two parts and riffling with the thumbs so the cards intermix
shuffling by splitting the pack and interweaving the two halves at their corners
Synonyms, antonyms and the broader and narrower relations for riffle come from Princeton University’s WordNet. Full sources and licences.