All 3 Uses

frivolous
in
Pride and Prejudice
(Auto-generated)

She lost all concern for him in finding herself thus selected as the object of such idle and frivolous gallantry; and while she steadily repressed it, could not but feel the reproof contained in his believing, that however long, and for whatever cause, his attentions had been withdrawn, her vanity would be gratified, and her preference secured at any time by their renewal.†
p. 225.3

frivolous = not serious
She has been allowed to dispose of her time in the most idle and frivolous manner, and to adopt any opinions that came in her way.†
p. 269.9 *
Allow me to say, Lady Catherine, that the arguments with which you have supported this extraordinary application have been as frivolous as the application was ill-judged.†
p. 338.1

Definitions:

(1) (frivolous)
not serious
- in behavior or attitude -- as when acting silly or without appropriate seriousness
- in content -- as when describing something as trivial or unimportant
(2) (meaning too rare to warrant focus)