WordNet (r) 3.0 (2006):

thick skin
    n 1: skin that is very thick (as an elephant or rhinoceros)

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "thick skin":
   Philistinism, analgesia, anesthesia, armature, armor, armor plate,
   body armor, buckler, bulletproof vest, callosity, callousness,
   callus, chain armor, chain mail, chitin, coat of mail, cortex,
   deadness, dullness, electronarcosis, elytron, episperm, flintiness,
   formidable defenses, habergeon, hard heart, hard shell,
   hardenedness, hardheartedness, hardness, hardness of heart,
   harness, hauberk, heart of stone, impassibility, imperception,
   imperceptiveness, imperceptivity, impercipience, imperviousness,
   inconsiderateness, induration, insensibility, insensibleness,
   insensitiveness, insensitivity, insentience, inuredness, lorica,
   lorication, mail, narcosis, narcotization, needles, numbness,
   obduracy, obdurateness, obtuseness, panoply, pericarp,
   pins and needles, plate, plate armor, protective covering,
   rhinoceros hide, scute, scutum, shell, shield, spines, stoniness,
   suit of armor, test, testa, unfeeling, unfeelingness,
   unperceptiveness