Facetious is used to describe something, such as a remark or behavior, that is meant to be humorous or funny but is sometimes instead annoying, silly, or improper. It can also be used to describe someone who is joking, often implying that they are doing so inappropriately.

// The emcee delivered several facetious quips throughout the night that the audience found in poor taste.

// I was just being facetious—I didn't mean it seriously.

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"In September, 1818, Byron told Moore of a new undertaking: 'It is called "Don Juan," and is meant to be a little quietly facetious upon every thing. ... I shall try the experiment, anonymously, and if it don't take it will be discontinued.' Safe to say that he continued, taking advantage of that freedom to cram into the poem pretty much anything that came to mind: shipwreck, cannibalism, lobster, cross-dressing, violent slurs upon the Duke of Wellington." — Anthony Lane, The New Yorker, 26 Feb. 2024

As many puzzle fans know, facetious is one of a small group of English words that not only use all five vowels once, but use them in alphabetical order. Other members of this exclusive club include abstemious (and abstemiously), and arsenious. (There is also an odd class of words which contain each vowel, used once, in reverse order: Pulmonifera, Muscoidea, and subcontinental.) Facetious comes from the Middle French adjective facetieux, which traces to the Latin word facētia, meaning "cleverness or wit." In English, it is used to describe speech or behavior that is intended to be playfully cheeky.