The Collaborative International Dictionary of English v.0.48:

Insane \In*sane"\, a. [L. insanus. See In- not, and Sane.]
   1. Exhibiting unsoundness or disorder of mind; not sane; mad;
      deranged in mind; delirious; distracted. See Insanity,
      2.
      [1913 Webster]

   2. Used by, or appropriated to, insane persons; as, an insane
      hospital.
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   3. Causing insanity or madness. [R.]
      [1913 Webster]

            Or have we eaten on the insaneroot
            That takes the reason prisoner ?      --Shak.
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   4. Characterized by insanity or the utmost folly; chimerical;
      unpractical; as, an insane plan, attempt, etc.
      [1913 Webster]

            I know not which was the insane measure. --Southey.
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WordNet (r) 3.0 (2006):

insane
    adj 1: afflicted with or characteristic of mental derangement;
           "was declared insane"; "insane laughter" [ant: sane]
    2: very foolish; "harebrained ideas"; "took insane risks behind
       the wheel"; "a completely mad scheme to build a bridge
       between two mountains" [syn: harebrained, insane, mad]

Moby Thesaurus II by Grady Ward, 1.0:

156 Moby Thesaurus words for "insane":
   abnormal, absurd, apish, asinine, at fever pitch, batty, bedlamite,
   befooled, beguiled, bereft of reason, besotted, bonkers, brainless,
   brainsick, buffoonish, buggy, bughouse, bugs, certifiable,
   cockeyed, crackbrained, cracked, crackers, cranky, crazed, crazy,
   credulous, cuckoo, daffy, daft, dazed, delirious, deluded,
   demented, deprived of reason, deranged, disordered, disoriented,
   distraught, dizzy, doting, dotty, dull, dumb, eccentric, fanatical,
   fatuitous, fatuous, febrile, feverish, flaky, flighty, fond, fool,
   foolheaded, foolish, frantic, frenetic, frenzied, fruity, fuddled,
   furious, futile, gaga, goofy, gulled, hallucinated, hectic,
   hysteric, idiotic, imbecile, imbecilic, inane, inept, infatuated,
   irrational, irresponsible, kinky, kooky, loco, loony, loopy,
   ludicrous, lunatic, mad, maddened, maniac, maniacal, manic,
   maudlin, mazed, mental, mentally deficient, meshuggah, mindless,
   moon-struck, moronic, neurotic, non compos, non compos mentis,
   nonsensical, not all there, not right, nuts, nutty, odd,
   of unsound mind, off, out of it, overanxious, overdesirous,
   overeager, overenthusiastic, overzealous, perfervid, potty, psycho,
   psychoneurotic, psychotic, queer, quirky, reasonless, reckless,
   ridiculous, round the bend, sappy, scatterbrained, schizo,
   schizoid, schizophrenic, screwy, senseless, sentimental, sick,
   silly, spaced out, spacy, stark-mad, stark-staring mad, strange,
   stupid, tetched, thoughtless, touched, ultrazealous, unbalanced,
   unhinged, unrealistic, unsane, unsettled, unsound, wacky,
   wandering, wet, wild, witless



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

INSANE. One deprived of the use of reason, after he has arrived at the age
when he ought to have it, either by a natural defect or by accident. Domat,
Lois Civ. Lib. prel. tit. 2, s. 1, n. 11.