Frivolity means foolish, or not serious.
Synonyms: lighthearted, "social butterfly", happy, silly, clown, whimsical, shallow.
Antonyms: serious, "stone faced", sensible, wise, understanding.
Examples of sentences using frivolity:
1.) The pompous spoiled princess, known for her frivolity, once bought all the red dresses from the surrounding stores so no one would ever look like her.
2.) The stone faced boy detested bringing his wide eyed frivolous sister along with him on hunting trips because she always complained that it was a shallow sport for unintelligent humans.