Antonyms of adj flippant

1 sense of flippant

Sense 1
flippant, light-minded -- (showing inappropriate levity)

INDIRECT (VIA frivolous) -> serious -- (concerned with work or important matters rather than play or trivialities; "a serious student of history"; "a serious attempt to learn to ski"; "gave me a serious look"; "a serious young man"; "are you serious or joking?"; "Don't be so serious!")

Similarity of adj flippant

1 sense of flippant

Sense 1
flippant, light-minded -- (showing inappropriate levity)
       => frivolous (vs. serious) -- (not serious in content or attitude or behavior; "a frivolous novel"; "a frivolous remark"; "a frivolous young woman")