The Collaborative International Dictionary of English v.0.48:

Inane \In*ane"\, a. [L. inanis.]
   Without contents; empty; void of sense or intelligence;
   purposeless; pointless; characterless; useless. "Vague and
   inane instincts." --I. Taylor. -- In*ane"ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Inane \In*ane"\, n.
   That which is void or empty. [R.]
   [1913 Webster]

         The undistinguishable inane of infinite space. --Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inane
    adj 1: devoid of intelligence [syn: asinine, fatuous,
           inane, mindless, vacuous]

Moby Thesaurus II by Grady Ward, 1.0:

254 Moby Thesaurus words for "inane":
   absurd, aimless, airy, apish, arid, asinine, awkward, bare, barren,
   batty, befooled, beguiled, besotted, blah, bland, blank,
   blankminded, bleached, bloodless, bonkers, bootless, brainless,
   buffoonish, callow, calm, catchpenny, characterless, clear,
   cockeyed, cold, colorless, counterproductive, crazy, credulous,
   cuckoo, daffy, daft, dazed, dead, designless, devoid, dilute,
   diluted, dippy, doting, dotty, draggy, drearisome, dreary,
   driveling, dry, dryasdust, dull, dumb, dusty, effete, elephantine,
   empty, empty-headed, empty-minded, empty-pated, empty-skulled,
   etiolated, fade, fatuitous, fatuous, featureless, feckless, flaky,
   flat, flavorless, flimsy, fond, fool, foolheaded, foolish, fribble,
   fribbling, frivolous, frothy, fruitless, fuddled, futile, gaga,
   garbled, gauche, goofy, green, groping, gruelly, gulled, heavy,
   ho-hum, hollow, idiotic, idle, ignorant, imbecile, imbecilic,
   importless, inadequate, incogitant, indifferent, ineffective,
   ineffectual, inefficacious, inept, inexcitable, inexperienced,
   infatuated, innocent, innocuous, inoperative, insane,
   insignificant, insipid, invalid, jejune, know-nothing, kooky,
   laughable, leaden, lifeless, light, loony, low-spirited, ludicrous,
   lunatic, mad, maudlin, meaningless, mild, milk-and-water, moronic,
   naive, namby-pamby, nescient, nirvanic, nonconnotative,
   nondenotative, nonsensical, nugacious, nugatory, null,
   null and void, nuts, nutty, oblivious, of no force, otiose, pale,
   pallid, pappy, passive, pedestrian, phatic, plodding, pointless,
   poky, ponderous, preposterous, pulpy, purportless, purposeless,
   quietistic, rattlebrained, rattleheaded, raw, relaxed, ridiculous,
   risible, sapless, sappy, savorless, scatterbrained, scrambled,
   screwy, senseless, sentimental, shallow, silly, simple, slender,
   slight, slow, solemn, spiceless, spiritless, stale, sterile, stiff,
   stodgy, strange to, stuffy, stupid, superficial, tasteless,
   tedious, tentative, thin, thoughtfree, thoughtless, tranquil,
   trifling, trite, trivial, unacquainted, unapprized, unavailing,
   uncomprehending, unconversant, unenlightened, unfamiliar,
   unflavored, unideaed, unilluminated, uninformed, uninitiated,
   unintellectual, unintelligent, unknowing, unlively, unmeaning,
   unoccupied, unposted, unreasonable, unreasoning, unrelieved,
   unripe, unsavory, unsignificant, unsure, unthinking, unversed,
   useless, vacant, vacuous, vain, vapid, void, wacky, washy, watered,
   watered-down, watery, weak, wet, white, windy, wishy-washy,
   with nothing inside, without content, witless, wooden