tough

adamant aggressive arduous awful bad brawny brawnier bullies bully burdensome callous chewy conscientious courageous crabby/crabbed crabbiest crabbiest crabby crabbier crabbier demanding difficult difficult dispassionate durable efficient energetic effortful exigent exacting feisty firm gangster forbidding formidable glutinous grievous gritty harder harder harder hard-nosed harsh hard hard hard harsher healthy hard-nosed/hardheaded harshest heaviest healthiest hardheaded high-grade heavy Herculean hood incompressible intractable intimidator insusceptible insensitive intractable/intransigent incompliant knotty leathery kinetic laborious knockabout lusty malevolent more efficient more demanding most primary more hard-nosed more hard-nosed more difficult more difficult more thoroughgoing most hardnosed most hardnosed naughty more durable more hardheaded more hardheaded more primary more thorough most crabbed most serious most thorough more crabbed muscular most hardheaded most hardheaded most hard nosed most hard nosed mucilaginous most thoroughgoing more intransigent most hard-nosed most hard-nosed most intractable most intractable most robust more intractable more intractable more aggressive more hard nosed more hard nosed more stringent more robust most awful most efficient most intransigent more hardnosed more hardnosed more serious nasty obdurate particular oppressive praiseworthy prisoner prisoners puissant primary/prime primest rascal remorseless robust rocky resilient roughneck reusable roughest rough-and-tumble rougher rough rugged rugged severe serious spunky staunchest stern strenuous staunch stony stalwart strictest stable sturdy stricter stringent stringy stubborn stiff stouter strict strong strong strong stout taxing tenacious thorny thick-skinned thoroughpaced tight tightest tiresome thorough/thoroughgoing troublesome tighter uncompromising unbendable uncompassionate unfeeling unhospitable trying unpitying tyrannical uncompliant unrelenting unsavory viscose vigorous vicious unstirred unsympathetic unyielding valorous viscous viscid uphill weighty wiry