I think “D” is not short for something, it is a representation of the moon.
The D is a suffix to the family name.
In normal Japanese, there are no middle names. In One Piece, this seems to still be the case.
Impact: This means that Luffy’s family name is “Monkey D.”, not “Monkey”. His name in English should be “Luffy Monkey D.”, not “Luffy D. Monkey”.
The D is not short for something.
When written in Japanese, the character that would be used to reference this sound in Japanese (ディ, pronounced Dī) is not used. The western character “D.” is used. This is a unique phenomenon in the show, as even characters with western names (Edward Newgate, Marshall Teach) have their names written out in katakana. I argue that this is because the way the character looks is more important than the way the character sounds, which falls into the next point.
The Kozuki and Shimotsuki clans are from the family of D.
We have never seen a “D.” in introduced in Wano. However, we have seen a couple of families with resemblant characters. Written as 光月, and 霜月 respectively, these families both contain the character 月 at the end of their names. How did I come upon this? Pure coincidence. In any case, this character, “radical 74” is a Kanji character that can be read as “moon” (tsuki). Why is this related or significant? Because of the origin/evolution of this character:
What did 月 look like thousands of years ago? Starkly similar to the western character “D.”
Could it be that the “月” is a version of the “D” that we are used to?
Random fun facts that can contribute to the theory.
In Chapter 154 where the will of D is first mentioned, the panel immediately afterwards features the moon prominently.
Kozuki Oden and Shimotsuki Yasuie (Ko D. Oden, Shimo D. Yasuie in the western world under this theory) both died with a smile on their faces.
Blackbeard as a child is depicted crying near a moon (perhaps the moon gives the D.’s the ability to transfer their wills?).
The families who overthrew the Ancient Kingdom so far have last names related to the sun, which is thematically often opposed to the moon. Nefertari is a name that refers to an ancient queen of Egypt (they only worshipped Aten, the sun disc). There is a famous work of art by Pablo Picasso entitled “Don Quixote” that prominently features the sun – if Oda is drawing from this, this would also be a sun reference. Could Nika the sun god be a god of these world nobles?
Impact: I’m not totally sure what this means. Could this be a reference to their origins? The capital of the kingdom? I don’t know.
Theory by yasuke1.