SERIOUS
dangerous, grave, grievous, serious, severe, life-threatening
(adjective) causing fear or anxiety by threatening great harm; “a dangerous operation”; “a grave situation”; “a grave illness”; “grievous bodily harm”; “a serious wound”; “a serious turn of events”; “a severe case of pneumonia”; “a life-threatening disease”
serious
(adjective) 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
(adjective) of great consequence; “marriage is a serious matter”
good, serious
(adjective) appealing to the mind; “good music”; “a serious book”
serious
(adjective) concerned with work or important matters rather than play or trivialities; “a serious student of history”; “a serious attempt to learn to ski”; “gave me a serious look”; “a serious young man”; “are you serious or joking?”; “Don’t be so serious!”
unplayful, serious, sober
(adjective) completely lacking in playfulness
Source: WordNet® 3.1
Etymology
Adjective
serious (comparative more serious or seriouser, superlative most serious or seriousest)
Without humor or expression of happiness; grave in manner or disposition
Synonyms: earnest, solemn
Important; weighty; not insignificant
Really intending what is said; in earnest; not jocular or deceiving
(of a relationship) Committed.
(East Africa): going to take action
Synonyms
• See also serious
Antonyms
• (important, weighty): trifling, unimportant
• (intending what is said): jesting
Adverb
serious (not comparable)
(colloquial, dialect) In a serious manner; seriously.
Source: Wiktionary
Se"ri*ous, a. Etym: [L. serius: cf. F. sérieux, LL. seriosus.]
1. Grave in manner or disposition; earnest; thoughtful; solemn; not
light, gay, or volatile.
He is always serious, yet there is about his manner a graceful ease.
Macaulay.
2. Really intending what is said; being in earnest; not jesting or
deceiving. Beaconsfield.
3. Important; weighty; not trifling; grave.
The holy Scriptures bring to our ears the most serious things in the
world. Young.
4. Hence, giving rise to apprehension; attended with danger; as, a
serious injury.
Syn.
– Grave; solemn; earnest; sedate; important; weighty. See Grave.
– Se"ri*ous*ly, adv.
– Se"ri*ous*ness, n.
Source: Webster’s Unabridged Dictionary 1913 Edition