1. (adj) serious
concerned with work or important matters rather than play or trivialities
Synonyms: unplayful, severe, grave, good, life-threatening, sober, dangerous, grievous
Antonyms: flighty, trivial, scatterbrained, light, lightheaded, light-headed, empty-headed, head-in-the-clouds, noncritical, silly, dizzy, flippant, flyaway, featherbrained, noncrucial, airheaded, frivolous, giddy, nonintellectual, easy, light-minded, unimportant, idle
2. (adj) serious
of great consequence
Synonyms: unplayful, severe, grave, good, life-threatening, sober, dangerous, grievous
Antonyms: dizzy, light, light-headed, empty-headed, flyaway, scatterbrained, giddy, lightheaded, head-in-the-clouds, nonintellectual, unimportant, flighty, light-minded, idle, flippant, airheaded, featherbrained, silly, trivial, noncrucial, easy, noncritical, frivolous
3. (adj) dangerous, grave, grievous, serious, severe, life-threatening
causing fear or anxiety by threatening great harm
Synonyms: severe, sober, serious, wicked, stark, unsafe, terrible, hard, spartan, stern, heavy, unplayful, weighty, monstrous, grave, good, grievous, atrocious, sedate, flagitious, life-threatening, dangerous, knockout, austere, heartbreaking, heartrending, solemn
Antonyms: light-headed, empty-headed, giddy, lightheaded, featherbrained, easy, dizzy, airheaded, flighty, flyaway, noncritical, head-in-the-clouds, trivial, frivolous, silly, unimportant, noncrucial, light, nonintellectual, flippant, light-minded, scatterbrained, idle
4. (adj) good, serious
appealing to the mind
Synonyms: dear, good, beneficial, unplayful, in effect(p), unspoiled, skillful, sound, secure, serious, effective, severe, upright, salutary, in force(p), skilful, practiced, dependable, estimable, life-threatening, grievous, expert, just, grave, dangerous, sober, proficient, full, honest, respectable, undecomposed, unspoilt, ripe, safe, adept, honorable, well(p), right, near
Antonyms: flighty, featherbrained, light-minded, scatterbrained, dizzy, flyaway, flippant, empty-headed, easy, noncritical, idle, airheaded, silly, noncrucial, lightheaded, light, head-in-the-clouds, unimportant, giddy, nonintellectual, light-headed, trivial, frivolous
5. (adj) unplayful, serious, sober
completely lacking in playfulness
Synonyms: serious, severe, sombre, drab, grave, good, sedate, unplayful, life-threatening, sober, somber, dangerous, grievous, solemn
Antonyms: dizzy, noncrucial, silly, giddy, flighty, lightheaded, empty-headed, flyaway, scatterbrained, flippant, light-minded, head-in-the-clouds, featherbrained, airheaded, light, trivial, unimportant, nonintellectual, light-headed, easy, noncritical, idle, frivolous
6. (adj) serious
requiring effort or concentration; complex and not easy to answer or solve
Synonyms: unplayful, severe, grave, good, life-threatening, sober, dangerous, grievous
Antonyms: light-headed, unimportant, nonintellectual, lightheaded, trivial, empty-headed, noncrucial, light, head-in-the-clouds, silly, flighty, noncritical, flippant, giddy, scatterbrained, flyaway, dizzy, light-minded, easy, featherbrained, frivolous, idle, airheaded
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Synonyms for serious 
