The Collaborative International Dictionary of English v.0.48:

Drain \Drain\ (dr[=a]n), v. t. [imp. & p. p. Drained
   (dr[=a]nd); p. pr. & vb. n. Draining.] [AS. drehnigean to
   drain, strain; perh. akin to E. draw.]
   1. To draw off by degrees; to cause to flow gradually out or
      off; hence, to cause the exhaustion of.
      [1913 Webster]

            Fountains drain the water from the ground adjacent.
                                                  --Bacon.
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            But it was not alone that the he drained their
            treasure and hampered their industry. --Motley.
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   2. To exhaust of liquid contents by drawing them off; to make
      gradually dry or empty; to remove surface water, as from
      streets, by gutters, etc.; to deprive of moisture; hence,
      to exhaust; to empty of wealth, resources, or the like;
      as, to drain a country of its specie.
      [1913 Webster]

            Sinking waters, the firm land to drain,
            Filled the capacious deep and formed the main.
                                                  --Roscommon.
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   3. To filter.
      [1913 Webster]

            Salt water, drained through twenty vessels of earth,
            hath become fresh.                    --Bacon.
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The Collaborative International Dictionary of English v.0.48:

Draining \Drain"ing\, vb. n. of Drain, v. t. (Agric.)
   The art of carrying off surplus water, as from land.
   [1913 Webster]

   Draining tile. Same as Draintile.
      [1913 Webster]

WordNet (r) 3.0 (2006):

draining
    adj 1: having a debilitating effect; "an exhausting job in the
           hot sun" [syn: draining, exhausting]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "draining":
   aspiration, bleeding, bloodletting, breakdown, broaching, clamant,
   clamorous, cleaning out, clearance, clearing, collapse,
   coming apart, crack-up, cracking, crumbling, crying, cupping,
   decadent, declining, defecation, degenerate, demanding, depletion,
   deteriorating, devitalizing, discharging cargo, disintegrating,
   drafting, drainage, drawing, drooping, dwindling, ebbing, effete,
   egress, elimination, emptying, enervating, enfeebling, evacuation,
   exacting, excretion, exhaustedness, exhausting, exhaustion,
   exigent, exorbitant, extortionate, fading, failing, falling,
   fatiguesome, fatiguing, flagging, fragmenting, going to pieces,
   grasping, grueling, importunate, insistent, instant, killing,
   languishing, loud, marcescent, milking, nervous exhaustion,
   nervous prostration, off-loading, persistent, pertinacious,
   phlebotomy, pining, pipetting, pressing, prostration, pumping,
   punishing, regressive, removal, retrograde, retrogressive, sapping,
   shriveling, sinking, siphoning, sliding, slipping, slumping,
   straining, stressful, subsiding, sucking, suction, tabetic,
   tapping, taxing, tiresome, tiring, toilsome, trying, unloading,
   urgent, venesection, venting, voidance, voiding, waning, wasting,
   weakening, weariful, wearing, wearisome, wearying, wilting,
   withering, worsening