The Collaborative International Dictionary of English v.0.48:

Insensible \In*sen"si*ble\, a. [L. insensibilis: cf. F.
   insensible. See In- not, and Sensible.]
   [1913 Webster]
   1. Destitute of the power of feeling or perceiving; wanting
      bodily sensibility; unconscious. --Milton.
      [1913 Webster]

   2. Not susceptible of emotion or passion; void of feeling;
      apathetic; unconcerned; indifferent; as, insensible to
      danger, fear, love, etc.; -- often used with of or to.
      [1913 Webster]

            Accept an obligation without being a slave to the
            giver, or insensible to his kindness. --Sir H.
                                                  Wotton.
      [1913 Webster]

            Lost in their loves, insensible of shame. --Dryden.
      [1913 Webster]

   3. Incapable of being perceived by the senses; imperceptible.
      Hence: Progressing by imperceptible degrees; slow;
      gradual; as, insensible motion.
      [1913 Webster]

            Two small and almost insensible pricks were found
            upon Cleopatra's arm.                 --Sir T.
                                                  Browne.
      [1913 Webster]

            They fall away,
            And languish with insensible decay.   --Dryden.
      [1913 Webster]

   4. Not sensible or reasonable; meaningless. [Obs.]
      [1913 Webster]

            If it make the indictment be insensible or
            uncertain, it shall be quashed.       --Sir M. Hale.

   5. Incapable of feeling a specific sensation or emotion; as,
      insensible to pity.
      [PJC]

   Syn: Imperceptible; imperceivable; dull; stupid; torpid;
        numb; unfeeling; apathetic; stoical; impassive;
        indifferent; unsusceptible; hard; callous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insensible
    adj 1: incapable of physical sensation; "insensible to pain";
           "insensible earth" [ant: sensible, sensitive]
    2: unaware of or indifferent to; "insensible to the suffering
       around him" [syn: insensible(p), unaffected(p)]
    3: barely able to be perceived; "the transition was almost
       indiscernible"; "an almost insensible change" [syn:
       indiscernible, insensible, undetectable]
    4: unresponsive to stimulation; "he lay insensible where he had
       fallen"; "drugged and senseless" [syn: insensible,
       senseless]

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "insensible":
   a stranger to, abiotic, absorbed, anesthetic, anesthetized,
   apathetic, asleep, azoic, behind the curtain, behind the scenes,
   benumbed, blind, blind to, blindfold, blindfolded, bloodless,
   blunt, callous, camouflaged, caught napping, cold, comatose,
   concealed, cool, dead, dead to, deadened, deaf, deaf to,
   dim-sighted, disguised, dispassionate, dull, dumb, emotionless,
   engrossed, exanimate, hard, hardened, hidden, impalpable,
   impassible, impassive, imperceptible, imperceptive, impercipient,
   impervious, imponderable, in ignorance of, inanimate, inanimated,
   inappreciable, incognizant, inconsiderate, indifferent,
   indiscernible, indurated, inert, insensate, insensible to,
   insensitive, insentient, intangible, intent, invisible, latent,
   lifeless, lost to, mind-blind, mindless, mute, myopic, napping,
   nearsighted, nonconceiving, nonconscious, nonliving,
   nonunderstanding, not with it, numb, numbed, obdurate, oblivious,
   obtuse, out, out cold, out of it, out of sight, pachydermatous,
   phlegmatic, preoccupied, purblind, rapt, rocky, secret, senseless,
   shortsighted, sightless, soulless, stoic, stolid, submerged,
   thick-skinned, thick-witted, thoughtless, torpid, unaffected,
   unanimated, unapparent, unappreciable, unapprehending, unaware,
   unaware of, unbeheld, unbeholdable, uncomprehending, unconscious,
   unconscious of, undiscernible, undiscerning, unfeeling, unfelt,
   unhearing, uninsightful, unknowing, unmindful, unmindful of,
   unmoved, unnoticed, unobservable, unobserved, unperceivable,
   unperceived, unperceiving, unperceptive, unprehensive, unrealized,
   unrealizing, unseeable, unseeing, unseen, unsusceptible,
   unsuspecting, untouched, unviewed, unwitnessed, unwitting,
   viewless, witless



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSENSIBLE. In the language of pleading, that which is unintelligible is
said to be insensible. Steph. Pl. 378.