cheer has 17 distinct synonyms spread across 5 of its 7 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. Its one recorded opposite is complain.
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.
cause (somebody) to feel happier or more cheerful
“She tried to cheer up the disappointed child when he failed to win the spelling bee”
the quality of being cheerful and dispelling gloom
“flowers added a note of cheerfulness to the drab room”
Synonyms, antonyms and the broader and narrower relations for cheer come from Princeton University’s WordNet. Full sources and licences.