This question already has an answer here:
- People's names as names for genitalia? 2 answers
Some people are named "Dick". What does "Dick" mean when it is a name?
Dick is a nickname for Richard. When it’s not a name, dick is slang for a penis or an unkind person. There are a lot of jokes about this.
From Behind the Name:
In the Middle Ages, it was common to use rhyming nicknames of this sort, many of which live on:
See Name Nerds for a list of these and other nickname patterns.
Back in the days, people used to shorten their names, Rick for Richard and use a nick name that rhymes with the shortened name, in this case, Dick. Other examples are: Edward-Ed-Ted. Robert-Rob-Bob....