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.

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Definitions of serious

serious (adj.)
concerned with work or important matters rather than play or trivialities;
a serious young man
are you serious or joking?
a serious student of history
Don't be so serious!
gave me a serious look
a serious attempt to learn to ski
serious (adj.)
of great consequence;
marriage is a serious matter
serious (adj.)
causing fear or anxiety by threatening great harm;
a serious wound
a serious turn of events
Synonyms: dangerous / grave / grievous / severe / life-threatening
serious (adj.)
appealing to the mind;
a serious book
Synonyms: good
serious (adj.)
requiring effort or concentration; complex and not easy to answer or solve;
raised serious objections to the proposal
the plan has a serious flaw
serious (adj.)
completely lacking in playfulness;
Synonyms: unplayful / sober