Crudely Drawn Cock - precisely what it says on the tin.
These dicks are often coloured magenta (purple) and created in a simplistic style using tools such as Microsoft paint.
They can be seen on the forums of www.b3ta.com where they have cult status and always achieve a cheap laugh. this may have been where they were first conceived.
i just photoshopped a cdc into the hand of george bush.