The Collaborative International Dictionary of English v.0.48:

Superman \Su"per*man`\, n.
   1. same as Overman, above.
      [Webster 1913 Suppl.]

   2. a fictional character of cartoons, movies and television,
      with superhuman powers such as great strength, the ability
      to fly, and x-ray vision. In the cartoon tale, he was born
      on the planet Krypton and sent by his parents into space
      before it exploded, and landed on earth, where he fights
      for "truth, justice, and the American way". He works
      incognito as a reporter at the Daily Planet, and is
      constantly trying to avoid the uncovering of his secret
      identity by a co-worker, Lois Lane.
      [PJC]

WordNet (r) 3.0 (2006):

superman
    n 1: a person with great powers and abilities [syn: demigod,
         superman, Ubermensch]
    2: street name for lysergic acid diethylamide [syn: acid,
       back breaker, battery-acid, dose, dot, Elvis,
       loony toons, Lucy in the sky with diamonds, pane,
       superman, window pane, Zen]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "Superman":
   Antaeus, Atlas, Briareus, Brobdingnagian, Charles Atlas, Cyclops,
   Goliath, Hercules, Polyphemus, Samson, Tarzan, Titan, bully,
   bullyboy, colossus, giant, gorilla, muscle man, powerhouse,
   stalwart, strong man, strong-arm man, the mighty, the strong,
   tough, tough guy, tower of strength