tough

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