I am writing an essay for In Our Time by Ernest Hemingway and I need a word that means "to lack seriousness".
I think it depends on what you consider as seriousness, like "depth" or serious mood.
Google translate also yields
flippant, glib, facetious, joking, jokey, lighthearted, fatuous, inane, senseless, thoughtless, flip; time-wasting, pointless, trivial, trifling, minor, petty, insignificant, unimportant, de minimis
Google response for lev·i·ty /ˈlevədē/ (noun): levity; (plural noun: levities) humor or frivolity, especially the treatment of a serious matter with humor or in a manner lacking due respect. i.e., "as an attempt to introduce a note of levity, the words were a disastrous flop"
synonyms: lightheartedness, high spirits, vivacity, liveliness, cheerfulness, cheeriness, humor, gaiety, fun, jocularity, hilarity, frivolity, amusement, mirth, laughter, merriment, glee, comedy, wit, wittiness, jollity, joviality, i.e., "without some occasional levity, the working environment is no better than a sweatshop"