frivolous

Synonyms: barmy childish dizzy empty-headed facetious featherbrained flighty flip flippant foolish gay giddy harebrained idiotic idle ill-considered impractical juvenile light light-minded minor niggling nonserious not serious paltry peripheral petty playful pointless puerile scatterbrained senseless shallow sportive superficial tongue-in-cheek unimportant unprofound volatile whimsical

Antonyms: grave mature sensible serious solemn thoughtful wise