The Collaborative International Dictionary of English v.0.48:

Impure \Im*pure"\, v. t.
   To defile; to pollute. [Obs.] --Bp. Hall.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Impure \Im*pure"\, a. [L. impurus; pref. im- not + purus pure:
   cf. F. impur. See Pure.]
   [1913 Webster]
   1. Not pure; not clean; dirty; foul; filthy; containing
      something which is unclean or unwholesome; mixed or
      impregnated extraneous substances; adulterated; as, impure
      water or air; impure drugs, food, etc.
      [1913 Webster]

   2. Defiled by sin or guilt; unholy; unhallowed; -- said of
      persons or things.
      [1913 Webster]

   3. Unchaste; lewd; unclean; obscene; as, impure language or
      ideas. "Impure desires." --Cowper.
      [1913 Webster]

   4. (Script.) Not purified according to the ceremonial law of
      Moses; unclean.
      [1913 Webster]

   5. (Language) Not accurate; not idiomatic; as, impure Latin;
      an impure style.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impure
    adj 1: combined with extraneous elements [ant: pure]
    2: (used of persons or behaviors) immoral or obscene; "impure
       thoughts" [ant: pure]
    3: having a physical or moral blemish so as to make impure
       according to dietary or ceremonial laws; "unclean meat"; "and
       the swine...is unclean to you"-Leviticus 11:3 [syn:
       unclean, impure] [ant: clean]