Source : Webster's Revised Unabridged Dictionary (1913)

Drain \Drain\, v. i.
   1. To flow gradually; as, the water of low ground drains off.

   2. To become emptied of liquor by flowing or dropping; as,
      let the vessel stand and drain.

Drain \Drain\, n.
   1. The act of draining, or of drawing off; gradual and
      continuous outflow or withdrawal; as, the drain of specie
      from a country.

   2. That means of which anything is drained; a channel; a
      trench; a water course; a sewer; a sink.

   3. pl. The grain from the mashing tub; as, brewers' drains.
      [Eng.] --Halliwell.

   {Box drain}, {Counter drain}. See under {Box}, {Counter}.

   {Right of drain} (Law), an easement or servitude by which one
      man has a right to convey water in pipes through or over
      the estate of another. --Kent.

Drain \Drain\, v. t. [imp. & p. p. {Drained}; 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.

            Fountains drain the water from the ground adjacent.
                                                  --Bacon.

            But it was not alone that the he drained their
            treasure and hampered their industry. --Motley.

   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.

            Sinking waters, the firm land to drain, Filled the
            capacious deep and formed the main.   --Roscommon.

   3. To filter.

            Salt water, drained through twenty vessels of earth,
            hath become fresh.                    --Bacon.

Source : WordNet®

drain
     n 1: emptying accomplished by draining [syn: {drainage}]
     2: tube inserted into a body cavity (as during surgery) to
        remove unwanted material
     3: a pipe through which liquid is carried away [syn: {drainpipe},
         {waste pipe}]
     4: a gradual depletion of energy or resources; "a drain on
        resources"; "a drain of young talent by emmigration"

drain
     v 1: flow off gradually; "The rain water drains into this big
          vat" [syn: {run out}]
     2: deplete of resources; "The exercise class drains me of
        energy"
     3: empty of liquid; drain the liquid from; "We drained the oil
        tank"
     4: make weak; "Life in the camp drained him" [syn: {enfeeble},
        {debilitate}]

Source : Free On-Line Dictionary of Computing

drain
     
        [IBM] Synonym for {flush}.  Has a connotation of finality
        about it; one speaks of draining a device before taking it
        off-line.
     
        [{Jargon File}]