WordWeb Online

1. Not serious in content, attitude or behaviour
   "a frivolous novel"; "a frivolous remark"; "a frivolous young woman"

See also: airheaded, dippy [informal], dizzy, earnestness, empty-headed, featherbrained, flighty, flippant, flyaway, giddy, head-in-the-clouds, idle, light, lightheaded, light-headed, light-minded, scatterbrained, serious-mindedness, seriousness, silly, sincerity, superficial, trivial

Antonym: serious