tough

fibrous, gristly, sinewy, stringy

brittle, crunchy, hard

flinty, leathery, resilient, stalwart

durable, enduring, everlasting, immortal, imperishable, lasting, permanent, stable, staunch (also stanch), staying, tenacious, unyielding

flourishing, prospering, thriving

able-bodied, brawny, muscular

fit, fortified, hale, healthy, husky, lusty, red-blooded, robust, sound, strapping, virile

annealed, seasoned, tempered

austere, bleak, comfortless, discomforting, forbidding, inhospitable, spartan, uncomfortable

rigorous, strict, stringent

agonizing, heartbreaking, heartrending, painful, wretched

crushing, grinding, overwhelming, wearing

insufferable, insupportable, intolerable, unbearable, unendurable

harrowing, tortuous

bad, disagreeable, hostile, unfriendly, unpleasant

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also 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

abstract, abstruse, complex, complicated, hard, intricate, involved, recondite, serious

problem, troublesome, troublous, vexatious, vexing, worrisome

burdensome, demanding, exacting, importunate, inconvenient, onerous, oppressive, painful, stressful

demanding, exacting

uncharitable, unforgiving

adamant, callous, hardened, hard-hearted, hard-line, immovable, implacable, inflexible, merciless, pitiless, relentless, rock-ribbed, stiff, unbending, uncompromising, unrelenting, unsparing, unyielding

dour, gruff

ascetic (also ascetical), monastic, monkish

browbeating, bullying