The Collaborative International Dictionary of English v.0.48:

Incoherent \In`co*her"ent\, a. [Pref. in- not + coherent: cf. F.
   incoh['e]rent.]
   [1913 Webster]
   1. Not coherent; wanting cohesion; loose; unconnected;
      physically disconnected; not fixed to each; -- said of
      material substances. --Woodward.
      [1913 Webster]

   2. Lacking logical coherence or agreement; incongruous;
      inconsistent; having no dependence of one part on another;
      logically disconnected; rambling; -- of speech or
      discourse. "The same rambling, incoherent manner." --Bp.
      Warburton.
      [1913 Webster]

   3. Exhibiting incoherent[2] speech or thought; -- of people;
      as, a confused and incoherent accident victim.
      [PJC]

   4. Lacking harmony or congruity of parts.
      [PJC]

WordNet (r) 3.0 (2006):

incoherent
    adj 1: without logical or meaningful connection; "a turgid
           incoherent presentation" [ant: coherent, consistent,
           logical, ordered]
    2: (physics) of waves having no stable definite or stable phase
       relation [ant: coherent]
    3: unable to express yourself clearly or fluently; "felt tongue-
       tied with embarrassment"; "incoherent with grief" [syn:
       incoherent, tongue-tied]

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "incoherent":
   Greek to one, abnormal, absurd, aleatoric, aleatory, aloof,
   ambiguous, amorphous, anomalous, apart, asunder, babbling,
   beyond understanding, bipartite, blobby, blurred, blurry, broad,
   broken, broken off, chance, chancy, chaotic, chopped-off, choppy,
   confused, decousu, delirious, detached, dichotomous, disconnected,
   discontinued, discontinuous, discordant, discrete, disjoined,
   disjointed, disjunctive, disordered, disproportionate, distinct,
   divergent, dizzy, episodic, fitful, foggy, fuzzy, gapped, garbled,
   general, giddy, hazy, herky-jerky, hit-or-miss, ill-defined,
   illogical, impenetrable, imprecise, in two, inaccurate,
   inarticulate, inchoate, incognizable, incommensurable,
   incommensurate, incompatible, incomprehensible, incongruous,
   inconsequent, inconsistent, inconsonant, indecisive, indefinable,
   indefinite, indeterminable, indeterminate, indistinct, inexact,
   inscrutable, insular, intermittent, interrupted, irrational,
   irreconcilable, irregular, jagged, jerky, jumbled, lax,
   lightheaded, loose, maundering, mixed up, muddled, nonadherent,
   nonadhesive, noncoherent, noncohesive, noncontinuous, nonlinear,
   nonsequential, nonserial, nonspecific, nonuniform, numinous,
   obscure, off, open, orderless, out of proportion, oxymoronic,
   paradoxical, parenthetic, partitioned, past comprehension, patchy,
   rambling, random, ranting, raving, scrambled, scrappy,
   self-contradictory, separate, shadowed forth, shadowy, shapeless,
   snatchy, spasmodic, spotty, stochastic, suspended, sweeping,
   tenuous, tongue-tied, unadhesive, unassociated, unattached,
   unattended, unclear, uncoherent, uncohesive, unconnected,
   undefined, undestined, undetermined, unfathomable, unintelligible,
   unjoined, unknowable, unorganized, unplain, unsearchable,
   unspecified, unsuccessive, untenacious, vague, veiled, wandering,
   wild