The Collaborative International Dictionary of English v.0.48:

Punish \Pun"ish\, v. t. [imp. & p. p. Punished; p. pr. & vb.
   n. Punishing.] [OE. punischen, F. punir, from L. punire,
   punitum, akin to poena punishment, penalty. See Pain, and
   -ish.]
   1. To impose a penalty upon; to afflict with pain, loss, or
      suffering for a crime or fault, either with or without a
      view to the offender's amendment; to cause to suffer in
      retribution; to chasten; as, to punish traitors with
      death; a father punishes his child for willful
      disobedience.
      [1913 Webster]

            A greater power
            Now ruled him, punished in the shape he sinned.
                                                  --Milton.
      [1913 Webster]

   2. To inflict a penalty for (an offense) upon the offender;
      to repay, as a fault, crime, etc., with pain or loss; as,
      to punish murder or treason with death.
      [1913 Webster]

   3. To injure, as by beating; to pommel. [Low]
      [1913 Webster]

   4. To deal with roughly or harshly; -- chiefly used with
      regard to a contest; as, our troops punished the enemy.
      [Colloq. or Slang]
      [Webster 1913 Suppl.]

   Syn: To chastise; castigate; scourge; whip; lash; correct;
        discipline. See Chasten.
        [1913 Webster]

WordNet (r) 3.0 (2006):

punishing
    adj 1: resulting in punishment; "the king imposed a punishing
           tax"
    2: characterized by effort to the point of exhaustion;
       especially physical effort; "worked their arduous way up the
       mining valley"; "a grueling campaign"; "hard labor"; "heavy
       work"; "heavy going"; "spent many laborious hours on the
       project"; "set a punishing pace" [syn: arduous,
       backbreaking, grueling, gruelling, hard, heavy,
       laborious, operose, punishing, toilsome]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "punishing":
   Herculean, arduous, backbreaking, burdensome, castigatory,
   chastening, chastising, corrective, crushing, demanding,
   disciplinary, draining, effortful, exhausting, fatiguesome,
   fatiguing, forced, grueling, hard, hard-earned, hard-fought, heavy,
   hefty, inflictive, killing, labored, laborious, onerous, operose,
   oppressive, painful, penal, penological, punitive, punitory,
   retributive, strained, straining, strenuous, stressful, taxing,
   tiresome, tiring, toilsome, torturous, tough, troublesome, trying,
   uphill, weariful, wearing, wearisome, wearying