The Collaborative International Dictionary of English v.0.48:

Feeble-minded \Fee"ble-mind"ed\, a.
   Weak in intellectual power; wanting firmness or constancy;
   irresolute; vacillating; imbecile. "comfort the
   feeble-minded." --1 Thess. v. 14. -- Fee"ble-mind"ed*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

feebleminded
    adj 1: retarded in intellectual development [syn: backward,
           half-witted, slow-witted, feebleminded]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "feeble-minded":
   boneheaded, deficient, dim-witted, dull-witted, dumb, empty-headed,
   exceptional, idiotic, imbecilic, mentally defective, moronic,
   retarded, simple, slow-witted, stupid, subnormal, thick, vacant,
   weak-minded, witless