The Collaborative International Dictionary of English v.0.48:

Simple-minded \Sim"ple-mind`ed\, a.
   Artless; guileless; simple-hearted; undesigning;
   unsuspecting; devoid of duplicity. --Blackstone. --
   Sim"ple-mind`ed*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

simple-minded
    adj 1: lacking subtlety and insight; "a simple-minded argument"
    2: lacking mental capacity and subtlety [syn: dim-witted,
       simple, simple-minded]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "simpleminded":
   arrested, artless, babbling, backward, blithering, bluff, blunt,
   born yesterday, burbling, candid, childlike, confiding,
   crackbrained, cracked, crazy, cretinistic, cretinous, dim-witted,
   direct, dithering, driveling, drooling, dull, feebleminded, frank,
   guileless, half-baked, half-witted, idiotic, imbecile, imbecilic,
   ingenu, ingenuous, innocent, maundering, mentally defective,
   mentally deficient, mentally handicapped, mentally retarded,
   mongoloid, moronic, naive, not all there, open, openhearted,
   outspoken, plain, retarded, simple, simplehearted, simpletonian,
   sincere, single-hearted, single-minded, slobbering, slow,
   slow-witted, subnormal, trustful, trusting, unguarded, unreserved,
   unsophisticated, unsuspicious, unwary