From Wiktionary, the free dictionary
From un- + frivolous.
unfrivolous (not comparable)
- Not frivolous
2009 January 23, Eve M. Kahn, “Conversation-Piece Buys, Maybe. Intriguing Histories, Definitely.”, in New York Times[1]:The annual Americana fest is meant to help collectors […] pore through unfrivolous, astringent, functional American design.