I think D stands for Dionysus the God of wine and madness. He symbolized freedom because during his festivals and rituals people let loose. His roman counterpart Liber Pater meant the Free Father.
- Oda has named characters after Gods. In this arc Prometheus, Zeus and Prince Loki were introduced.
- The D clan is linked to the Void Century and Ancient Kingdom. The Ancients Weapons were named after Graeco-Roman gods. It would fit in that mold.
When characters are named after Gods small parts of their mythology are incorporated.
- Prometheus is a homie made out of fire. Prometheus brought humans fire.
- King Neptune is the ruler of an underwater kingdom and his called God of the Sea. Neptune is the god of the sea.
I believe two D members incorporate bits of Dionysus’ myth and that is Ace and Law.
- Ace used the sock and buskin in his jolly roger and on his hat. Dionysus’ festivals were the driving force behind the development of Greek theatre. Ace was a fire logia. Dinoysus was engulfed in flames while in the womb. Ace was targeted for death while still in the womb, and his mother carried him for twenty months. Hera tried to orchestrate Dionysus’ death but he survived and Zeus carried him is his thigh to full term.
- Law’s clothes incorporate leopard spots. Dionysus wore leopard skins and famously rode on leopards. Law created created satyrs and centaurs. Dionysus retinue included satyrs and was a student of Chiron. Law is associated a lot with hearts. One of Dionysus’ myths is that he was ripped to pieces and his heart was used to restore him. Law can cut people into pieces. Dionysus go to punishment were to have people ripped apart and maenads during their ritual madness would tear apart animals to pieces.
I see more of Dionysus’ myth in Law and that is because I see him having more incentive to discover the Will of D. than Luffy. The World Government is largely responsible for the calamity that befell his country. Also it was through Law’s flashback we got important info on the Will of D.
*Theory by ecass305