Longman Dictionary of Contemporary English - impure

~ adj 1 an impure substance has something else mixed with it, especially something of a lower quality  (An added danger was that the group was using impure sodium chlorate in their bombs.)  (- opposite pure (1)) 2 old-fashioned or humorous impure thoughts, feelings etc are morally bad, especially because they are about sex  (He tried, without success, to rid his mind of any impure thoughts about Julia.)  (- opposite pure (4))