Definition of serious :
1. Grave in manner or disposition; earnest; thoughtful; solemn; not light, gay, or volatile.
2. Hence, giving rise to apprehension; attended with danger; as, a serious injury.
3. Important; weighty; not trifling; grave.
4. Really intending what is said; being in earnest; not jesting or deceiving.
laborious, of import, in earnest, effective, intellectual, upright, no-nonsense, austere, life-threatening, heavy, somber, unspoiled, help, near, dear, respectable, unsafe, in force, grievous, wicked, practiced, just, terrible, knockout, businesslike, sobersided, sedate, staid, easy, uphill, sober, tall, drab, severe, expert, sobering, sound, atrocious, thoughtful, spartan, full, weighty, sombre, good, difficult, sincere, safe, real, honest, estimable, flagitious, overserious, skillful, honorable, skilful, pressing, stark, sensible, right, beneficial, in effect, heartbreaking, solemn, secure, hard, salutary, unplayful, monstrous, attitude, undecomposed, proficient, heartrending, arduous, work, ripe, adept, capital, unspoilt, dependable, well, dangerous
adult (part of speech: adjective)
resolute (part of speech: adjective)
implacable, unwavering, valiant, decided, indomitable, steady, dauntless, decisive, unflinching, stubborn, willing, resolute, dogged, thorough, resolved, hearty, steadfast, unchangeable, iron-willed, constant, true, plucky, insistent, unyielding, unshakeable, bold, devoted, loyal, stern, strong-minded, relentless, diligent, steely, indefatigable, courageous, persistent, determined, tenacious, committed, tough, bullheaded, ruthless, uncompromising, staunch, reliable, faithful, firm, deliberate, zealous, set, unbending, tireless, headstrong, earnest, intent, inexorable, dedicated, strong-willed, spunky, persevering, fearless
important (part of speech: adjective)
grave, world-shaking, momentous, high-powered, pivotal, crucial, big-name, important, distinguished, big-wig, influential, considerable, earthshaking, consequential, noteworthy, prominent, salient, critical, notable, heavyweight, eminent