The Collaborative International Dictionary of English v.0.48:

Fruitless \Fruit"less\, a.
   1. Lacking, or not bearing, fruit; barren; destitute of
      offspring; as, a fruitless tree or shrub; a fruitless
      marriage. --Shak.
      [1913 Webster]

   2. Productive of no advantage or good effect; vain; idle;
      useless; unprofitable; as, a fruitless attempt; a
      fruitless controversy.
      [1913 Webster]

            They in mutual accusation spent
            The fruitless hours.                  --Milton.

   Syn: Useless; barren; unprofitable; abortive; ineffectual;
        vain; idle; profitless. See Useless. --
        Fruit"less*ly, adv. -- Fruit"lness*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fruitless
    adj 1: 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:

91 Moby Thesaurus words for "fruitless":
   abortive, acarpous, arid, barren, bootless, celibate, childless,
   counterproductive, desert, desolate, drained, dried-up, dry,
   effete, empty, etiolated, exhausted, failed, failing, fallow,
   fatuitous, fatuous, feckless, foiled, frustrated, futile, gainless,
   gaunt, gelded, idle, impotent, inadequate, inane, ineffective,
   ineffectual, inefficacious, infecund, infertile, inoperative,
   invalid, issueless, jejune, lame, leached, manque, menopausal,
   miscarried, miscarrying, nonfertile, nonproducing, nonproductive,
   nonprolific, nonremunerative, nugacious, nugatory, of no effect,
   of no force, otiose, pointless, profitless, rewardless, sine prole,
   sterile, stickit, stillborn, successless, sucked dry, teemless,
   thwarted, unavailable, unavailing, uncultivated, unfertile,
   unfortunate, unfruitful, unplowed, unproductive, unprofitable,
   unprolific, unremunerative, unrewarding, unsown, unsuccessful,
   untilled, useless, vain, virgin, waste, wasted, without issue,
   worthless