Cage is on the money:
Back in the days of Old and Middle English, everything was written by hand; “it was therefore common and easier to use agreed-upon abbreviations.” An example of an abbreviation was “Ric.” for “Richard”.
The abbreviations led to diminutives, such as “Rick”.
Rhyming nicknames were fairly common in the 12th and 13th centuries, such as “Hick” and “Dick” from “Rick”. “Dick and Hick were among the earliest of the rhyming nicknames, first appearing in writing around 1220. Other rhyming nicknames include Polly from Molly, Bob from Rob (from Robert), Bill from Will (from William); and Hodge from Roger.”
Dick has remained a nickname for Richard. ”‘Hick’ has thankfully become obsolete, except when tied to ‘Dick’ in rhymes such as ‘Hickory, Dickory, Dock.’”
From: http://answers.google.com/answers/threadview?id=61502