Merriam-Webster Online Dictionary
serious (adjective)1.
thoughtful or subdued in appearance or manner - sober a quiet, serious girl
2.
a) requiring much thought or work - serious study
b) of or relating to a matter of importance - a serious play
3.
a) not joking or trifling being in earnest - a serious question
b) archaic - pious
c) deeply interested - devoted a serious musician
4.
a) not easily answered or solved - serious objections
b) having important or dangerous possible consequences - a serious injury
5.
excessive or impressive in quality, quantity, extent, or degree - serious stereo equipment making serious money serious drinking
Merriam-Webster Online Thesaurus
serious (adjective)1.
not joking or playful in mood or manner
SYNONIMS:
earnest, grave, humorless, no-nonsense, po-faced, sedate, severe, sober, sobersided, solemn, staid, uncomic, unsmiling, weightyRELATED WORDS:
harsh, stern, strict; businesslike, professional; dignified, distinguished, elevated, serious-minded; gloomy, grimNEAR ANTONYMS:
antic, comic, comical, droll, farcical, funny, hilarious, hysterical ( hysteric), laughable, light, light-headed, ludicrous, ridiculous, riotous, risible, screaming, sidesplitting, uproarious; featherbrained, flighty, frivolous, goofy, harebrained, lighthearted, puerile, scatterbrained; absurd, asinine, balmy, brainless, cockeyed, crazy, cuckoo, daffy, daft, dotty, fatuous, foolish, half-witted, insane, jerky, kooky ( kookie), loony ( looney), lunatic, mad, nonsensical, nutty, preposterous, sappy, screwball, senseless, silly, unwise, wacky ( whacky), weak-minded, witless, zany2.
having a matter of importance as its topic
SYNONIMS:
grave, heavy, weightyRELATED WORDS:
big, consequential, eventful, important, major, material, meaningful, momentous, portentous, significant, solid, substantialNEAR ANTONYMS:
frivolous, insignificant, lite, little, Mickey Mouse, minor, silly, slight, small, trivial, unimportant3.
involving potential loss or injury
SYNONIMS:
grave, grievous, hazardous, jeopardizing, menacing, parlous, perilous, risky, serious, threatening, unhealthy, unsafe, venturesomeRELATED WORDS:
dicey, insecure, precarious, treacherous, uncertain; ultrahazardous; chance, haphazard, random; distressing, sickening, unpleasant; ugly, wicked; adverse, bad, baleful, baneful, deleterious, detrimental, evil, harmful, hurtful, ill, inimical, injurious, malignant, nasty, noxious, pernicious, pestilent; deadly, deathly, destructive, dire, fatal, fateful, fell, killer, lethal, mortal, murderousNEAR ANTONYMS:
advantageous, beneficial, good; ultrasafe