The adjective frivolous means someone who behaves in a silly or light-hearted way, rather than being serious and sensible. This also means something silly or not important, rather than useful or serious.
Synonyms are foolish, idiotic, trivial, or petty.
The word origins from Latin (15th century) frivolus meaning “silly, empty, trifling, worthless”, diminutive of frivos meaning “broken, crumbled”, and from friare meaning “break, rub away, crumble”.
Her prayers may seem frivolous but it means a lot to him.
I might take a break to try doing something completely frivolous.
He doesn’t see her as a frivolous young woman.