Link: https://onepiece.fandom.com/wiki/Will_of_D.

The "Will of D." is a mystery surrounding some people who have the initial "D." in their names.

The initial was first pointed out in Oda's SBS questions and answers corner, when he was asked what the D. in Luffy's name stood for. Oda just replied to read it as a D. for now and that he would reveal the truth behind it in time. This was the first time the D. in Luffy's name was brought up, as well as Oda's first hint toward a bigger mystery.