The Collaborative International Dictionary of English v.0.48:

Tough \Tough\, a. [Compar. Tougher; superl. Toughest.] [OE.
   tough, AS. t[=o]h, akin to D. taai, LG. taa, tage, tau, OHG.
   z[=a]hi, G. z[aum]he, and also to AS. getenge near to, close
   to, oppressive, OS. bitengi.]
   1. Having the quality of flexibility without brittleness;
      yielding to force without breaking; capable of resisting
      great strain; as, the ligaments of animals are remarkably
      tough. "Tough roots and stubs. " --Milton.
      [1913 Webster]

   2. Not easily broken; able to endure hardship; firm; strong;
      -- of objects and people; as, tough sinews. --Cowper.
      [1913 Webster]

            A body made of brass, the crone demands, . . .
            Tough to the last, and with no toil to tire.
                                                  --Dryden.
      [1913 Webster]

            The basis of his character was caution combined with
            tough tenacity of purpose.            --J. A.
                                                  Symonds.
      [1913 Webster]

   3. Not easily separated; viscous; clammy; tenacious; as,
      tough phlegm.
      [1913 Webster]

   4. Stiff; rigid; not flexible; stubborn; as, a tough bow.
      [1913 Webster]

            So tough a frame she could not bend.  --Dryden.
      [1913 Webster]

   5. Severe; violent; as, a tough storm. [Colloq.] " A tough
      debate. " --Fuller.
      [1913 Webster]

   6. Difficult to do, perform, or accomplish; as, a tough job.
      [PJC]

   7. Prone to aggressive or violent behavior; rowdyish; -- of
      people, or groups; as, a tough neighborhood; a tough
      character.
      [PJC]

   To make it tough, to make it a matter of difficulty; to
      make it a hard matter. [Obs.] --Chaucer.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

tough \tough\, n.
   A person who is tough[7]; a ruffian; a thug; as, a cluster of
   neighborhood toughs hanging out on the corner.
   [PJC]

WordNet (r) 3.0 (2006):

tough
    adj 1: not given to gentleness or sentimentality; "a tough
           character" [ant: tender]
    2: very difficult; severely testing stamina or resolution; "a
       rugged competitive examination"; "the rugged conditions of
       frontier life"; "the competition was tough"; "it's a tough
       life"; "it was a tough job" [syn: rugged, tough]
    3: physically toughened; "the tough bottoms of his feet" [syn:
       tough, toughened] [ant: tender, untoughened]
    4: substantially made or constructed; "sturdy steel shelves";
       "sturdy canvas"; "a tough all-weather fabric"; "some plastics
       are as tough as metal" [syn: sturdy, tough]
    5: violent and lawless; "the more ruffianly element"; "tough
       street gangs" [syn: ruffianly, tough]
    6: feeling physical discomfort or pain (`tough' is occasionally
       used colloquially for `bad'); "my throat feels bad"; "she
       felt bad all over"; "he was feeling tough after a restless
       night" [syn: bad, tough]
    7: resistant to cutting or chewing [ant: tender]
    8: unfortunate or hard to bear; "had hard luck"; "a tough break"
       [syn: hard, tough]
    9: making great mental demands; hard to comprehend or solve or
       believe; "a baffling problem"; "I faced the knotty problem of
       what to have for breakfast"; "a problematic situation at
       home" [syn: baffling, elusive, knotty, problematic,
       problematical, tough]
    n 1: someone who learned to fight in the streets rather than
         being formally trained in the sport of boxing [syn: street
         fighter, tough]
    2: an aggressive and violent young criminal [syn: hood,
       hoodlum, goon, punk, thug, tough, toughie,
       strong-armer]
    3: a cruel and brutal fellow [syn: bully, tough, hooligan,
       ruffian, roughneck, rowdy, yob, yobo, yobbo]

Moby Thesaurus II by Grady Ward, 1.0:

501 Moby Thesaurus words for "tough":
   Antaeus, Atlas, Briareus, Brobdingnagian, Charles Atlas, Cyclops,
   Goliath, Herculean, Hercules, Mafioso, OK, Polyphemus, Samson,
   Spartan, Spartanic, Superman, Tarzan, Titan, Young Turk, abiding,
   abstruse, ace-high, acute, adamant, adamantine, adherent, adhesive,
   age-long, aged, amylaceous, ancient, antique, arbitrary, arduous,
   astringent, athletic, austere, authoritarian, backbreaking, bad,
   baffling, bang-up, battler, beast, beldam, belligerent,
   belted knight, berserk, berserker, beyond one, bickerer, blade,
   bomber, bony, bonzer, boss, bravo, brawler, brawny, bruiser,
   brutal, brute, bulky, bulldogged, bulldoggish, bulldoggy,
   bullheaded, bully, bullyboy, burdensome, burly, but good, callous,
   cartilaginous, cement, cemental, chewy, chronic, clabbered, clammy,
   clingy, clotted, coagulated, cohesive, cold, colossus, combatant,
   competitor, complex, complicated, concrete, confirmed, constant,
   contender, contestant, continuing, cool, coriaceous, corking,
   corneous, crabbed, crackerjack, cramp, critical, crushing, curdled,
   cutting, dandy, delicate, delicious, demanding, demon, dense,
   devil, diamondlike, difficile, difficult, disputant, diuturnal,
   doughty, doughy, dour, dragon, drastic, ducky, duelist, durable,
   dure, effortful, enduring, enforcer, evergreen, exacting,
   excessive, exigent, exorbitant, extravagant, extreme, fab, fencer,
   feuder, fibrous, fiend, fierce, fighter, fighting cock,
   fine and dandy, fire-eater, firebrand, firm, fixed, flintlike,
   flinty, foilsman, forced, formidable, furious, fury, game,
   gamecock, gamy, garbled, gaumy, gear, gelatinous, giant, gladiator,
   glairy, gluelike, gluey, glutenous, glutinose, glutinous, gooey,
   goon, gorilla, granitelike, granitic, great, grim, gristly, gritty,
   groovy, grueling, grumous, gumbo, gumbolike, gumlike, gummous,
   gummy, gun, gunsel, gutsy, gutty, hairy, hard, hard as nails,
   hard to understand, hard-bitten, hard-boiled, hard-earned,
   hard-fought, hard-line, hard-nosed, hard-set, hardened,
   hardhearted, hardnose, hardy, harsh, hatchet man, heavy, hefty,
   hell-raiser, hellcat, hellhound, hellion, holy terror, hood,
   hoodlum, hooligan, horny, hot, hothead, hotspur, hunky-dory, icy,
   immoderate, immutable, incendiary, inflexible, inspissated,
   intemperate, intense, intractable, intransient, intrepid,
   intricate, inured, inveterate, irksome, iron-hard, ironlike,
   jam-up, jawbreaking, jelled, jellied, jellylike, jouster, jumbled,
   just dandy, keen, killer, killing, knight, knotted, knotty,
   labored, laborious, lapideous, lasting, leatherlike, leathery,
   lithoid, lithoidal, long-lasting, long-lived, long-standing,
   long-term, longeval, longevous, macrobiotic, mad dog, madcap,
   manly, marble, marblelike, marvy, massive, mean, meticulous,
   mettlesome, militant, monster, mucilaginous, mucker, mug, mugger,
   muscle man, muscular, mystifying, narrow, neat, nervy, nifty,
   no picnic, nobby, not easy, obdurate, obfuscated, obscure,
   obscured, obstinate, of long duration, of long standing, okay,
   onerous, operose, oppressive, osseous, out of sight, outrageous,
   overtechnical, painful, pasty, peachy, peachy-keen, perdurable,
   perduring, perennial, permanent, perpetual, perplexed, perplexing,
   persistent, persisting, pertinacious, piercing, plucky, plug-ugly,
   powerful, powerhouse, procrustean, punishing, punk, puzzling,
   quarreler, rapist, red-blooded, refractory, remaining, resistant,
   resistive, resolute, revolutionary, rigid, rigorous, rioter,
   ripping, rival, rocklike, rocky, rodman, ropy, rough, roughneck,
   rowdy, ruffian, rugged, rum, sabreur, savage, scrambled, scrapper,
   scrumptious, scuffler, self-adhesive, sempervirent,
   set with thorns, severe, sharp, she-wolf, sinewy, slabby, slap-up,
   slimy, slithery, smashing, solid, something else, sound, spiffing,
   spiffy, spiny, spirited, spiritful, spitfire, splitting, spunky,
   squabbler, stable, stalwart, starchy, staying, steadfast, steady,
   steellike, steely, steep, stern, stickable, sticky, stiff, stodgy,
   stonelike, stony, stout, strained, strapping, strenuous, strict,
   stringent, stringy, strong, strong arm, strong man, strong-arm man,
   strong-armer, struggler, stubborn, stunning, sturdy, substantial,
   swashbuckler, swell, sword, swordplayer, swordsman, syrupy, tacky,
   taut, taxing, tenacious, termagant, terror, terrorist, the mighty,
   the strong, thick, thickened, thorny, thug, ticklish, tiger,
   tigress, tilter, toilsome, torpedo, tough as leather, tough guy,
   tower of strength, tremelloid, tremellose, tricky, trigger man,
   troublesome, tussler, ugly customer, unalterable, unbending,
   unbreakable, uncaring, uncompromising, unconscionable,
   underprivileged, unfading, unfeeling, ungiving, unsentimental,
   unsparing, unsympathetic, untiring, unyielding, uphill, vehement,
   venomous, vigorous, violent, virago, virile, virulent, viscid,
   viscose, viscous, vital, vixen, wearisome, well-built,
   well-constructed, well-founded, well-grounded, well-made, wicked,
   wild beast, wiry, witch, withstanding, wizard, wolf, wrangler,
   yahoo