cjrgreen: Rhyming nicknames, possibly. Richard -> Rick -> Dick.
"Dick" as a nickname for "Richard" is very old, known from the 13th C.
"Dick" as a vulgarity is much more recent, 19th C.
Aniketos: Well I figured that much out but didn't know the 13th C. Where'd you hear that?
According to Cecil of
The Straight Dope, "Dick" as a name is known in writing from 1220. (Cecil's pretty good about sources, but didn't give one this time.)
A better-known, though later, use is Shakespeare. "
Prince Henry: 'Sirrah, I am sworn brother to a leash of drawers; and can call them all by their christen names, as Tom, Dick, and Francis.'" (Henry IV, Part I: ii.4: 1597)
In that quote, though, Shakespeare has pulled off an elaborate double entendre: a "leash of drawers" can be taken to mean not only the bartenders with whom Henry spends his idleness and his father's gold, but also the drawstring of his drawers and the three manifestations of his masculinity found within. So Shakespeare actually might have been the first surviving use of "Dick" in
that sense, though he had such a gift for using common current speech in his work that the expression already may have been well known.