Chapter 1115 told us that Joyboy is the first one to be called a pirate. However in current timeline, Luffy has shown us everything else except being a real pirate. So Joyboy 900 years ago was sailing a normal ship and doing what Luffy was doing which is to bring freedom and laughter to everyone. From my speculation, I think the symbol that more suited for the Joyboy was a smiley mouth.
In Alabasta arc, all of the members share the same vision of Joyboy that was so extraordinary that led to creating a symbol as the mark of friendship. The same event probably happened to Nefertari Lily from what happened to Vivi.
If the smiley mouth is drawn on the left hand like this, when it lift up will be come the letter 'D'. But the downside is that the ink will fade away, so they put the letter 'D' into their name to make sure they will always find each other.
I am not 100% sure how the 20 kingdoms found out the letter 'D' is what distinct themselves as the crew of liberation. They just label them as their enemies, thus comes the statement of "D is the enemy of God".
