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.

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

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. [1913 Webster]

3. Causing insanity or madness. [R.]

[1913 Webster]

Or have we eaten on the insaneroot That takes the reason prisoner ? --Shak. [1913 Webster]

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. [1913 Webster]

Source: The Collaborative International Dictionary of English v.0.48
 

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

Source: Moby Thesaurus II by Grady Ward, 1.0
 

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, mad]

Source: WordNet (r) 2.0
 

Insane \In*sane"\, a. [L. insanus. See In- not, and Sane.]

1. Exhibiting unsoundness or disorded of mind; not sane; mad; deranged in mind; delirious; distracted. See Insanity, 2.

2. Used by, or appropriated to, insane persons; as, an insane hospital.

3. Causing insanity or madness. [R.]

Or have we eaten on the insaneroot That takes the reason prisoner ? --Shak.

4. Characterized by insanity or the utmost folly; chimerical; unpractical; as, an insane plan, attempt, etc.

I know not which was the insane measure. --Southey.

Source: Webster's Revised Unabridged Dictionary (1913)