humdrum has 7 distinct synonyms spread across 3 of its 3 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.
not challenging; dull and lacking excitement
“an unglamorous job greasing engines”
tediously repetitious or lacking in variety
“a humdrum existence; all work and no play”
the quality of wearisome constancy, routine, and lack of variety
“he had never grown accustomed to the monotony of his work”
Synonyms, antonyms and the broader and narrower relations for humdrum come from Princeton University’s WordNet. Full sources and licences.