Notes:

If any of you are interested in the real answer, Thrillist covers it pretty well

"Turns out, calling someone D--- instead of Richard is a somewhat recent evolution of the nicknames Rich and Rick, both of which we also use instead of Richard to this day. As the video's host explains, it's sort of like one of those "knee bone connected to the thy bone" progressions. In the days of writing everything by hand, Rich and Rick emerged as common nicknames for Richard, and apparently, people also used to like to come up with nicknames that rhyme. This gave rise to nicknames like D--- and Hick around the early Thirteenth Century, but unlike Hick, D--- continues to stick around today.

Of course, D--- isn't the only nickname to emerge from a similar rhyme-based progression. For example, have you ever wondered by people named Robert are often called Bob? "