The Collaborative International Dictionary of English v.0.48:

feeble-mindedness \fee"ble-mind"ed*ness\ n.
   severe mental deficiency.

   Syn: amentia.
        [WordNet 1.5]

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]