The Collaborative International Dictionary of English v.0.48:

Leach \Leach\, n. [Written also letch.] [Cf. As. le['a]h lye,
   G. lauge. See Lye.]
   1. A quantity of wood ashes, through which water passes, and
      thus imbibes the alkali.
      [1913 Webster]

   2. A tub or vat for leaching ashes, bark, etc.
      [1913 Webster]

   Leach tub, a wooden tub in which ashes are leached.
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The Collaborative International Dictionary of English v.0.48:

Lecher \Lech"er\, n. [OE. lechur, lechour, OF. lecheor, lecheur,
   gormand, glutton, libertine, parasite, fr. lechier to lick,
   F. l['e]cher; of Teutonic origin. See Lick.]
   A man given to lewdness; one addicted, in an excessive
   degree, to the indulgence of sexual desire, or to illicit
   sexual relations with women; also called letch and lech.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Letch \Letch\ (l[e^]ch), v. & n.
   See Leach.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Letch \Letch\, n. [See Lech, Lecher.]
   1. Strong desire; passion; especially, lust. [Archaic]
      [1913 Webster]

            Some people have a letch for unmasking impostors, or
            for avenging the wrongs of others.    --De Quincey.
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   2. A lecher. [slang]
      [PJC]

WordNet (r) 3.0 (2006):

letch
    n 1: man with strong sexual desires [syn: satyr, lecher,
         lech, letch]