Herculean, abstruse, ardent, arduous, assiduous, backbreaking, brutal, burdensome, careful, complex, critical, crushing, delicate, demanding, detailed, determined, difficile, difficult, diligent, dogged, effortful, energetic, exacting, exhausting, exhaustive, fatiguing, fervent, forced, formidable, grueling, hairy, hard, hard-earned, hard-fought, hard-working, hardworking, heavy, hefty, indefatigable, industrious, intricate, jawbreaking, killing, knotted, knotty, labored, mean, never idle, no picnic, not easy, obdurate, obstinate, onerous, operose, oppressive, overworked, painful, painstaking, persevering, persistent, ponderous, punishing, relentless, rigorous, rough, rugged, scrupulous, sedulous, set with thorns, severe, sleepless, spiny, steadfast, steady, steep, stiff, strained, strenuous, stubborn, taxing, thorny, thorough, ticklish, tireless, tiring, toilsome, tough, tricky, troublesome, unflagging, unrelenting, unremitting, unsleeping, unsparing, untiring, unwavering, unwearied, uphill, vehement, wearisome, wearying, wicked, zealous