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):

fruitlessly
    adv 1: in an unproductive manner [syn: unproductively,
           fruitlessly, unprofitably] [ant: fruitfully,
           productively, profitably]