I'm happy to see that I'm not the only one wondering this

Dutch (grand)parents?
Makes sense but still doesn’t make sense!

The name existed long before it became the slang for penis and also used to mean other things in conversation.

The way those old short-names are made is by taking the short of one's name, say Robert is Rob, then finding a rhyming name, Rob turn into Bob.

Richard would have a short of Rick. Rick rhymes with Dick.
Ta da.

Yeah guys, I have feelings too

Because that what they are

Cos dick is the short form of Richard