1. (a.) Grave in manner or disposition; earnest; thoughtful; solemn; not light, gay, or volatile.

2. (a.) Really intending what is said; being in earnest; not jesting or deceiving.

3. (a.) Important; weighty; not trifling; grave.

4. (a.) Hence, giving rise to apprehension; attended with danger; as, a serious injury.

abandoned acute afire alarming ardent arduous austere awe-inspiring bad bound bound and determined burning businesslike cogitative cognitive committed comprehensive concentrating concentrative conceptive conceptual conceptualized consequential considerable contemplating contemplative critical crucial dangerous dangersome decided decisive decorous dedicated deep definite deliberating deliberative demure determined devoted devout difficult dignified dour earnest elevated excogitating exhaustive explosive faithful fatal fateful fell fervent fervid fiery flaming formal formidable fraught with danger frowning full genuine grand grave great grievous grim grim-faced grim-visaged hearty heated heavy honest hot hot-blooded humorless ideative impassioned important imposing in earnest inspiring intense intent intent on introspective irresistible jeopardous laborious life-and-death life-or-death lofty long-faced loyal main majestic major maximum meditating meditative menacing mental mighty moderate momentous moving museful musing nasty no joke no laughing matter no-nonsense noble noetic obstinate on fire operose parlous passionate pensive perfervid periculous perilous persevering persistent plenary poker-faced pondering portentous powerful precarious prehensive pressing purposeful red-hot reflecting reflective relentless resolute resolved ruminant ruminating ruminative sedate serious-minded severe significant sincere single-minded sober sober-minded sobersided solemn somber speculative spirited staid stately steady stern stone-faced straight-faced straightforward strenuous strong sublime temperate tenacious thinking thought thoughtful threatening total tough ugly unamusing unfunny unsmiling urgent vehement vital warm weighty white-hot wholehearted wistful zealous