tough

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