The substance is just "SAD" not "S.A.D". Considering that Caesar Clown is the only person in the world who knows how to manufacture it, and he uses it to create artificial fruits that he calls "smiles", it was just an appropriate name to fit into the theme. Clown, smiles, sad, etc.  

![SAD vat][1]


  [1]: http://vignette4.wikia.nocookie.net/onepiece/images/c/c2/SAD_Infobox.png/revision/latest/scale-to-width-down/250?cb=20130915135855