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.
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            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.
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            Sinking waters, the firm land to drain,
            Filled the capacious deep and formed the main.
                                                  --Roscommon.
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   3. To filter.
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            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.
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   Draining tile. Same as Draintile.
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WordNet (r) 3.0 (2006):

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