The Collaborative International Dictionary of English v.0.48:

Gibberish \Gib"ber*ish\, a.
   Unmeaning; as, gibberish language.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

gibberish \gib"ber*ish\ (j[i^]b"b[~e]r*[i^]sh or
   g[i^]b"b[~e]r*[i^]sh), n. [From Gibber, v. i.]
   1. Rapid and inarticulate talk; unintelligible language;
      unmeaning words.
      [1913 Webster]

            He, like a gypsy, oftentimes would go;
            All kinds of gibberish he had learnt to know.
                                                  --Drayton.
      [1913 Webster]

            Such gibberish as children may be heard amusing
            themselves with.                      --Hawthorne.
      [1913 Webster]

   2. Incomprehensible, obscure, or pretentious technical talk
      or writing; excessively obscure jargon.
      [PJC]

WordNet (r) 3.0 (2006):

gibberish
    n 1: unintelligible talking [syn: gibberish, gibber]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "gibberish":
   Aesopian language, Babel, Gongorism, Greek, abracadabra, absurdity,
   amphigory, argot, babble, babblement, balderdash, balls,
   bibble-babble, blabber, blather, blether, bombast, bullshit, bunk,
   bunkum, cackle, cant, chatter, cipher, claptrap, code, crap,
   cryptogram, double Dutch, double-talk, drivel, drool,
   fiddle-faddle, fiddledeedee, flummery, folderol, fudge, fustian,
   gabble, galimatias, gammon, garbage, garble, gibber, gibble-gabble,
   gift of tongues, glossolalia, gobbledygook, hocus-pocus, humbug,
   jabber, jabberwocky, jargon, jumble, lingo, magic, mumbo jumbo,
   mumbo-jumbo, mummery, narrishkeit, niaiserie, noise, nonsense,
   pack of nonsense, palaver, patois, patter, phraseology, piffle,
   poppycock, prate, prattle, rant, rigamarole, rigmarole,
   rodomontade, rubbish, scatology, scramble, secret language,
   skimble-skamble, slang, sorcery, stuff and nonsense,
   stultiloquence, taboo language, thaumaturgy, trash, tripe,
   trumpery, twaddle, twattle, twiddle-twaddle, vaporing, vernacular,
   vocabulary, vulgar language, waffling