WordWeb Online

1. Resulting in punishment
   "the king imposed a punishing tax"
    
2. Characterized by effort to the point of exhaustion; especially physical effort
   "set a punishing pace"
   - arduous, backbreaking, grueling [US], gruelling [Brit, Cdn], hard, heavy, laborious, operose, toilsome

1. Impose a penalty on; inflict punishment on
   "we had to punish the dog for soiling the floor again"
   - penalize, penalise [Brit]

See also: effortful, punishingly, punishment, punitive

Encyclopedia: Punish