Dick-tec-tive. ""fellow, lad, man," 1553, rhyming nickname for Rick, short for Richard, one of the commonest Eng. names, They couldn't spell. ... names, it has long been a synonym for "fellow," and so most of the slang senses are probably very old, but naturally hard to find in the surviving records. Meaning "dick" is recorded from 1908, perhaps as a shortened variant of detective. cowboy...

It comes from the story of the famous detective Dick Tracy.

in general , dick was derived from prick my dictionary said . and dick with the meaning of "detective" might be derived from shortening & alteration of detective. according to online webster check it up on 3 http://www.merriam-webster.com/dictionary/dick