Matthew Arnold is to blame. He is the one who concocted “high seriousness” as the ultimate standard of greatness in English poetry.1 And by now, “serious” has almost become the name of a subgenre. There is “serious fiction,” and there is everything else. Practically everybody understands that “serious” is not the opposite of “humorous,” but rather of “frivolous.” What, however, does the word itself mean?
It comes from the Latin serius for “grave”; and at first it seems to have meant just that—sorrowful, sad, fretful. (Dickens three hundred years after it began to appear widely in English speech contrasted serious to smirking.) Since then it seems to have diverged into two different traditions of meaning. One is headed by St. Thomas More, for whom “serious” was a synonym of “earnest.” The other is headed by Sir Thomas Elyot, who spoke in The Governour of Socrates’ “serious disciples.” In this tradition—by far the ascendant one today—“serious” became a term of rather vague approbation. The disciple, for instance, is praised for his seriousness because he has nothing else worth praising.
The term is vague because it has no natural opposite. What is unserious? In fact, serious is opposed, in any plain man’s vocabulary, to humorous, to pleasurable; the serious employments of man are on one side, wearing dark colors and a frown; the light and trifling amusements are on the other side. When a plain man says, “Are you serious?” he means, “You’re not joking, are you?” He doesn’t mean: “Is what you are saying of lasting importance; is it weighty; does it require considerable thought; is it spoken with fervent intent?”
In the first chapter of Book V of Tom Jones, Fielding equates seriousness with dullness. The English pantomime, he writes,
This was, perhaps, no very civil use of such personages; but the contrivance was, nevertheless, ingenious enough, and had its effect. And this will now plainly appear, if, instead of serious and comic, we supply the words duller and dullest; for the comic was certainly duller than anything before shown on stage, and could be set off only by the superlative degree of dullness which composed the serious.
1. “[T]he superiority of poetry over history consists in its possessing a higher truth and a higher seriousness. . .” (“The Study of Poetry”  in Selections from the Prose Works of Matthew Arnold, ed. William Savage Johnson [Boston: Houghton Mifflin, 1913], p. 88).