The Collaborative International Dictionary of English v.0.48:

Futile \Fu"tile\ (?; 277), a. [L. futilis that easily pours out,
   that easily lets loose, vain, worthless, from the root of
   fundere to pour out: cf. F. futile. See Fuse, v. t.]
   1. Talkative; loquacious; tattling. [Obs.]
      [1913 Webster]

            Talkers and futile persons.           --Bacon.
      [1913 Webster]

   2. Of no importance; having no useful purpose; useless; vain;
      worthless; pointless. "Futile theories." --I. Taylor.
      [1913 Webster]

            His reasoning . . . was singularly futile.
                                                  --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

futile
    adj 1: producing no result or effect; "a futile effort"; "the
           therapy was ineffectual"; "an otiose undertaking"; "an
           unavailing attempt" [syn: futile, ineffectual,
           otiose, unavailing]
    2: unproductive of success; "a fruitless search"; "futile years
       after her artistic peak"; "a sleeveless errand"; "a vain
       attempt" [syn: bootless, fruitless, futile,
       sleeveless, vain]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "futile":
   abortive, absurd, airy, apish, asinine, bad, barren, batty,
   befooled, beguiled, besotted, bootless, brainless, buffoonish,
   catchpenny, cockeyed, counterproductive, crazy, credulous, daffy,
   daft, dazed, dizzy, doomed, doting, dumb, effete, empty, etiolated,
   failed, failing, fatuitous, fatuous, feckless, flaky, flimsy, fond,
   fool, foolheaded, foolish, foredoomed, fribble, fribbling,
   frivolous, frothy, fruitless, fuddled, gaga, goofy, gulled, hollow,
   idiotic, idle, ill-advised, ill-considered, ill-suited, ill-timed,
   imbecile, impolitic, impotent, improper, in vain, inadequate,
   inadvisable, inane, inappropriate, inapt, incongruous, ineffective,
   ineffectual, inefficacious, inefficient, inept, inexpedient,
   infatuated, infelicitous, inoperative, inopportune, insane,
   insufficient, invalid, kooky, lame, light, loony, mad,
   mal a propos, malapropos, manque, maudlin, miscarried, miscarrying,
   moronic, nugacious, nugatory, nutty, of no effect, of no force,
   otiose, out of place, profitless, sappy, screwy, senseless,
   sentimental, shallow, silly, slender, slight, sterile, stickit,
   stillborn, stupid, successless, superficial, thoughtless, trifling,
   trite, trivial, unavailable, unavailing, unbefitting, undesirable,
   unfit, unfitting, unfortunate, unhappy, unmeet, unproductive,
   unprofitable, unsatisfactory, unseasonable, unseemly, unsuccessful,
   unsuitable, untimely, unwise, useless, vacuous, vain, vapid, wacky,
   wet, windy, witless, worthless, wrong