onerous

Synonyms: arduous austere backbreaking burdensome crushing cumbersome demanding difficult distressing embittering exacting excessive exhausting exigent fatiguing formidable galling grave grinding grueling hard harsh headache heavy intolerable irksome laborious merciless oppressive overpowering overtaxing painful plodding ponderous pressing responsible rigorous serious severe strenuous taxing tiresome tiring toilsome troublesome vexatious weighty

Antonyms: common easy light trivial