The Collaborative International Dictionary of English v.0.48:

Senseless \Sense"less\, a.
   Destitute of, deficient in, or contrary to, sense; without
   sensibility or feeling; unconscious; stupid; foolish; unwise;
   unreasonable.
   [1913 Webster]

         You blocks, you stones, you worse than senseless
         things.                                  --Shak.
   [1913 Webster]

         The ears are senseless that should give us hearing.
                                                  --Shak.
   [1913 Webster]

         The senseless grave feels not your pious sorrows.
                                                  --Rowe.
   [1913 Webster]

         They were a senseless, stupid race.      --Swift.
   [1913 Webster]

         They would repent this their senseless perverseness
         when it would be too late.               --Clarendon.
   [1913 Webster] -- Sense"less*ly, adv. -- Sense"less*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

senseless
    adj 1: not marked by the use of reason; "mindless violence";
           "reasonless hostility"; "a senseless act" [syn:
           mindless, reasonless, senseless]
    2: unresponsive to stimulation; "he lay insensible where he had
       fallen"; "drugged and senseless" [syn: insensible,
       senseless]
    3: serving no useful purpose; having no excuse for being;
       "otiose lines in a play"; "advice is wasted words"; "a
       pointless remark"; "a life essentially purposeless";
       "senseless violence" [syn: otiose, pointless,
       purposeless, senseless, superfluous, wasted]
    4: (of especially persons) lacking sense or understanding or
       judgment [syn: nitwitted, senseless, soft-witted,
       witless]

Moby Thesaurus II by Grady Ward, 1.0:

295 Moby Thesaurus words for "senseless":
   abiotic, abnormal, absonant, absurd, aimless, amorphous,
   anesthetized, apish, asinine, asleep, azoic, batty, befooled,
   beguiled, benumbed, bereft of reason, besotted, brainless,
   brainsick, buffoonish, capricious, casual, cataleptic, catatonic,
   cockeyed, cold, comatose, contradictory, contrary to reason,
   crackbrained, cracked, crazed, crazy, credulous, daffy, daft,
   dazed, dead, deadened, deluded, demented, deprived of reason,
   deranged, designless, desultory, dippy, disarticulated,
   discontinuous, disjunct, disordered, disoriented, dispersed,
   disproportionate, distraught, dizzy, doped, doting, drugged, dumb,
   empty, empty-headed, erratic, exanimate, fallacious, fatuitous,
   fatuous, faulty, fitful, flaky, flawed, flighty, fond, fool,
   foolheaded, foolish, formless, frivolous, fuddled, futile, gaga,
   garbled, goofy, gratuitous, gulled, half-conscious, hallucinated,
   haphazard, headless, hit-or-miss, idiotic, ignorant, ill-advised,
   ill-considered, ill-contrived, ill-devised, ill-gauged, ill-judged,
   illogical, imbecile, imbecilic, immethodical, impolitic,
   importless, imprudent, inadvisable, inane, inanimate, inanimated,
   inauthentic, inchoate, incoherent, inconclusive, incongruous,
   inconsequent, inconsequential, inconsiderate, inconsistent,
   indiscreet, indiscriminate, inept, inert, inexpedient, infatuated,
   injudicious, insane, insensate, insensible, insensitive,
   insentient, insignificant, invalid, irrational, irregular, kooky,
   lackbrained, lean-minded, lean-witted, lifeless, loco, loony,
   loose, ludicrous, lunatic, mad, maddened, manic, maudlin, mazed,
   meaningless, mental, mentally deficient, meshuggah, mindless,
   misadvised, misguided, misshapen, moon-struck, moronic, mute,
   myopic, narcotized, nirvanic, nitwitted, non compos,
   non compos mentis, nonconnotative, nonconscious, nondenotative,
   nonliving, nonscientific, nonsensical, nonsymmetrical,
   nonsystematic, nonuniform, not all there, not bright,
   not following, not right, null, numbed, nutty, oblivious, odd,
   of little brain, of unsound mind, off, orderless, out, out cold,
   out of it, paralogical, pea-brained, phatic, pin-brained, planless,
   pointless, promiscuous, psycho, purportless, purposeless, queer,
   random, reasonless, reckless, ridiculous, sappy, scrambled, screwy,
   self-annulling, self-contradictory, self-refuting, semiconscious,
   sentimental, shapeless, shortsighted, sick, silly, slackminded,
   slackwitted, soulless, spaced out, spasmodic, sporadic, stark-mad,
   stark-staring mad, stoned, straggling, straggly, strange,
   strung out, stunned, stupid, surd, systemless, tetched,
   thoughtless, touched, trinal, unadvised, unanimated, unarranged,
   unauthentic, unaware, unbalanced, unclassified, unconnected,
   unconscious, unconsidered, undirected, undiscerning, unfeeling,
   unforeseeing, ungifted, ungraded, unhinged, unimportant,
   unintellectual, unintelligent, unjoined, unmeaning, unmethodical,
   unordered, unorganized, unphilosophical, unreasonable, unreasoning,
   unreflecting, unreflective, unsane, unscientific, unseeing,
   unsensible, unsettled, unsignificant, unsorted, unsound,
   unsymmetrical, unsystematic, untalented, unthinking, unthoughtful,
   ununiform, unwise, unwitty, vague, wacky, wandering, weak-minded,
   wet, wild, without reason, witless, wooden, zonked, zonked out