rigorous

austere, authoritarian, flinty, hard, harsh, heavy-handed, ramrod, rigid, rigorous, stern, strict, tough

demanding, exacting; uncharitable, unforgiving; adamant, adamantine, callous, hardened, hard-hearted, hard-line, immovable, implacable, inflexible, merciless, ossified, pitiless, relentless, rock-ribbed, stiff, unbending, uncompromising, unrelenting, unsparing, unyielding; dour, gruff; ascetic ( ascetical), monastic, monkish; browbeating, bullying; determined, firm, resolved, single-minded, steadfast, unflinching; dogged, intractable, obstinate, relentless

easy, easygoing, laid-back, undemanding; charitable, kind, merciful, mild, patient, soft, softhearted; accepting, compromising, yielding; responsive, willing; acquiescent, agreeable, amenable, complaisant, compliant, flexible, pliable, pliant

accurate, close, delicate, exact, fine, hairline, mathematical, pinpoint, refined, rigorous, spot-on

correct, right, strict, true; definite, definitive; nice, subtle; careful, fastidious, finical, finicky, meticulous

approximate, round; false, incorrect, untrue, wrong; careless, loose; indefinite, unclear, vague; doubtful, dubious, questionable, unreliable, untrustworthy

brassbound, cast-iron, exacting, hard-line, inflexible, rigorous, strict, stringent, uncompromising

close, conscientious, exact, fussy, meticulous, painstaking, punctilious, scrupulous, undeviating; adamant, adamantine, determined, dogged, firm, relentless, resolved, single-minded, steadfast, stubborn, tenacious, unbending, unflinching; immovable, implacable, unappeasable, unrelenting, unsparing, unyielding; austere, demanding, flinty, grim, hard, hardened, hard-hearted, harsh, ironbound, severe, stern, tough

acquiescent, compliant, compromising, pliable, pliant, relenting, yielding; easy, easygoing, gentle, indulgent, kindly, lenient, merciful, mild, pampering, soft, spoiling, tolerant; neglectful, negligent, remiss, slipshod, sloppy, slovenly, unfussy

arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, formidable, grueling ( gruelling), heavy, hellacious, herculean, killer, laborious, moiling, murderous, pick-and-shovel, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphill

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic ( problematical), recondite, serious, spiny, stubborn, thorny, ticklish, tricky; bruising, burdensome, exhausting, labored, onerous, oppressive, stressful, taxing, tight, trying; annoying, bothersome, distressing, irksome, troublesome, vexatious; grievous, grim, strict, stringent; brutal, cruel, inhuman, painful

achievable, clear, doable, elementary, manageable, uncomplicated; comforting, gentle, painless, relaxed, smooth, soothing; accessible, friendly, idiotproof, user-friendly