tough

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