The Collaborative International Dictionary of English v.0.48:

Thick-skinned \Thick"-skinned`\, a.
   Having a thick skin; hence, not sensitive; dull; obtuse.
   --Holland.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thick-skinned
    adj 1: insensitive to criticism [syn: thick-skinned, tough-
           skinned]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "thick-skinned":
   callous, case-hardened, dull, hard-boiled, hardened, impervious,
   insensate, insensitive, inured, numbed, obtuse, pachydermatous,
   steeled, stolid, tough, toughened, unfeeling, unsusceptible