Etimology
From Middle English seryows, from Old French serieux, from Medieval Latin sēriōsus, an extension of Latin sērius (“grave, earnest, serious”), from Proto-Indo-European *swer- (“heavy”). Cognate with German schwer (“heavy, difficult, severe”), Old English swǣr (“heavy, grave, grievous”). More at swear, sweer.
adjective
serious (comparative more serious, superlative most serious)
Without humor or expression of happiness; grave in manner or disposition
Important; weighty; not insignificant
Really intending what is said (or planned, etc); in earnest; not jocular or deceiving
(of a relationship) Committed.
Examples
Synonyms: earnest, solemn
deadly serious
It was a surprise to see the captain, who had always seemed so serious, laugh so heartily.
This is a serious problem. We'll need our best experts.
After all these years, we're finally getting serious attention.
He says he wants to buy the team, but is he serious?
Related words
synonyms
See also Thesaurus:serious
antonyms
(antonym(s) of "important, weighty"): trifling, unimportant
(antonym(s) of "intending what is said"): jesting
adverb
serious (not comparable)
(colloquial or dialect) seriously, in a serious manner (most often heard in take or mean serious)
Examples
The only time I walk out on singin' is when there's muckin' about and youse don't take it serious.