unserious
/ʌnˈsɪərɪəs
/adjective
Not serious; light-hearted: he defended himself with an unserious disingenuousness...
- It sometimes fails to be recognised that he is seriously unserious.
- It's an unserious allegation that merits no serious response.
- One of the panellists - who's kind of a friend of mine - says I shouldn't say ‘cool’ because it makes me sound unserious.