meaninglessness has 4 distinct synonyms spread across 1 of its 2 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 meaningfulness.
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 message that seems to convey no meaning
the quality of having no value or significance
“he resented the meaninglessness of the tasks they assigned him”
Synonyms, antonyms and the broader and narrower relations for meaninglessness come from Princeton University’s WordNet. Full sources and licences.