Grievous

Adjective

 * 1) Causing grief, pain or sorrow
 * 2) * 1883, Robert Louis Stevenson, Treasure Island
 * As for the captain, his wounds were grievous indeed but not dangerous.
 * 1) serious, grave, dire or dangerous

Adverbs for Grievous
deplorably; lamentably; sadly; piteously distressingly; intolerably; appallingly; horribly; unaccountably; unwarrantably; irremediably; unutterably; uncontrollably; bitterly; touchingly; genuinely; grimly; sorely; cruelly; inexpressibly; inordinately; terribly.

Synonyms for Grievous
lamentable, painful, afflicting, heavy, baleful, unhappy, woeful, sorrowful, oppressive, burdensome, distressing, weighty, serious, mortal, momentous, outrageous, heinous, calamitous.

Antonyms for Grievous
light, trivial, venial, harmless, salutary, advantageous, innocuous, good, pleasant, pleasurable, gratifying, agreeable, comforting, beneficial.

Thesaurus
abominable, acute, affecting, afflictive, aggrieved, anguished, appalling, arrant, atrocious, awful, base, beastly, beneath contempt, bitter, black, blameworthy, bleak, brutal, burdensome, calamitous, careworn, cataclysmal, cataclysmic, catastrophic, cheerless, comfortless, contemptible, damaging, dangerous, demanding, deplorable, depressing, depressive, despicable, destructive, detestable, dire, disastrous, discomforting, disgusting, dismal, dismaying, distasteful, distressful, distressing, doleful, dolorific, dolorogenic, dolorous, dreadful, dreary, dumb with grief, egregious, enormous, exacting, exigent, fatal, fell, fetid, filthy, flagrant, foul, fulsome, galling, grave, grief-stricken, griefful, grieved, gross, harmful, hateful, heartbreaking, heartrending, heavy, heinous, horrible, horrid, hurtful, in grief, infamous, intolerable, joyless, lamentable, loathsome, lousy, lugubrious, major, monstrous, mournful, moving, nasty, nefarious, noisome, notorious, obnoxious, odious, offensive, oppressive, outrageous, painful, pathetic, piteous, pitiable, pitiful, plaintive, plangent, plunged in grief, poignant, rank, regrettable, reprehensible, repulsive, rotten, rueful, ruinous, sad, saddening, scandalous, schlock, scurvy, serious, severe, shabby, shameful, sharp, shocking, shoddy, sordid, sore, sorrowed, sorrowful, sorrowing, squalid, superincumbent, taxing, tearful, terrible, too bad, touching, tough, tragic, ugly, unbearable, unclean, uncomfortable, unfortunate, unpalatable, vile, villainous, weighty, woebegone, woeful, worst, worthless, wounding, wreckful, wretched

Alternative forms

 * greuous

Etymology
From grieve, from the greven, from the  grever, from  gravare (which means to burden). Developed in the 13th century.