The Collaborative International Dictionary of English v.0.48:

Exhaust \Ex*haust"\, v. t. [imp. & p. p. Exhausted; p. pr. &
   vb. n. Exhausting.] [L. exhaustus, p. p. of exhaurire; ex
   out + haurire, haustum, to draw, esp. water; perhaps akin to
   Icel. asua to sprinkle, pump.]
   1. To draw or let out wholly; to drain off completely; as, to
      exhaust the water of a well; the moisture of the earth is
      exhausted by evaporation.
      [1913 Webster]

   2. To empty by drawing or letting out the contents; as, to
      exhaust a well, or a treasury.
      [1913 Webster]

   3. To drain, metaphorically; to use or expend wholly, or till
      the supply comes to an end; to deprive wholly of strength;
      to use up; to weary or tire out; to wear out; as, to
      exhaust one's strength, patience, or resources.
      [1913 Webster]

            A decrepit, exhausted old man at fifty-five.
                                                  --Motley.
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   4. To bring out or develop completely; to discuss thoroughly;
      as, to exhaust a subject.
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   5. (Chem.) To subject to the action of various solvents in
      order to remove all soluble substances or extractives; as,
      to exhaust a drug successively with water, alcohol, and
      ether.
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   Exhausted receiver. (Physics) See under Receiver.

   Syn: To spend; consume; tire out; weary.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Exhausting \Ex*haust"ing\, a.
   Producing exhaustion; as, exhausting labors. --
   Ex*haust"ing, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exhausting
    adj 1: having a debilitating effect; "an exhausting job in the
           hot sun" [syn: draining, exhausting]
    2: producing exhaustion; "an exhausting march"; "the visit was
       especially wearing" [syn: exhausting, tiring, wearing,
       wearying]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "exhausting":
   arduous, burdensome, crippling, debilitating, devitalizing,
   difficult, draining, enervating, enfeebling, fatiguesome,
   fatiguing, grinding, grueling, hard, harsh, heavy, killing,
   laborious, onerous, oppressive, overburdensome, punishing, sapping,
   straining, strenuous, stressful, tiresome, tiring, toilsome,
   trying, tyrannous, weakening, weariful, wearing, wearisome,
   wearying, weighty