serious (adj.)

mid-15c., "expressing earnest purpose or thought" (of persons), from Old French serios "grave, earnest" (14c., Modern French sérieux) and directly from Late Latin seriosus, from Latin serius "weighty, important, grave," probably from a PIE root *sehro- "slow, heavy" (source also of Lithuanian sveriu, sverti "to weigh, lift," svarus "heavy, weighty;" Old English swær "heavy," German schwer "heavy," Gothic swers "honored, esteemed," literally "weighty"). As opposite of jesting, from 1712; as opposite of light (of music, theater, etc.), from 1762. Meaning "attended with danger" is from 1800.