Thesaurus

Synonyms for Grievous include:

acute, all important, all-important, alpha, anguished, atrocious, bad, bereaved, bereft, beta, big, bitter, black, brokenhearted, burning, cardinal, central, chief, consequential, Copernican, corruptive, critical, crucial, dangerous, dark, demonic, desperate, despicable, devilish, diabolic, diabolical, dire, distinguished, dolorous, dolourous, elegiac, eminent, essential, eventful, evil, evil-minded, fiendish, flagitious, fundamental, grand, grave, great, grief-stricken, grieving, grievous, heartbreaking, heartbroken, heartrending, heartsick, heavy, heinous, hellish, historic, immoral, important, in-chief, infernal, joyless, key, lachrymose, lamenting, life-threatening, lugubrious, main, measurable, mephistophelean, mephistophelian, monstrous, most-valuable, mournful, mourning, of import, of the essence, offensive, outstanding, perversive, pestiferous, plaintive, primal, primary, principal, sad, satanic, serious, severe, significant, sinister, slimy, sorrowful, sorrowing, strategic, tearful, tormented, tortured, ugly, unhappy, unholy, unrighteous, unworthy, valuable, vile, wailful, wailing, weeping, weighty, wicked, woebegone, woeful, worthless, wretched, wrong

grievous is a ADJ.