The Collaborative International Dictionary of English v.0.48:

blithering \blithering\ adj.
   talking incoherently; as, a blithering idiot.

   Syn: jabbering.
        [WordNet 1.5]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "blithering":
   absolute, arrested, babbling, backward, blasted, blessed, burbling,
   crackbrained, cracked, crazy, cretinistic, cretinous, dithering,
   downright, driveling, drooling, gross, half-baked, half-witted,
   idiotic, imbecile, imbecilic, maundering, mentally defective,
   mentally deficient, mentally handicapped, mentally retarded,
   mongoloid, moronic, not all there, out-and-out, outright, positive,
   rank, retarded, simple, simpleminded, simpletonian, slobbering,
   subnormal