According to Christopher Lee’s autobiography, the name "Dooku" comes from the Japanese word for poison, "doku.". To many, it sounds a lot like "dookie.". A slang word for feces. Count Jard Dooku is the full name of Dooku.
To become a Sith Lord, Dooku had to murder someone close to him.
http://www.thegeektwins.com/2012/04/fourteen-things-you-probably-didnt-know.html
14 Things You Probably Didn't Know About Count Dooku