When searching, remember, a few words are better than many
ID4-Soap or White Dragon
Question: I sometimes hear players naming a “White Dragon” a “Soap”. Where did this name come from?
Answer: Prior to the “White Dragon” tile having a rectangular shape around it, it was a blank white tile. Legend has it that many years ago players began referring to it as a soap because the little white tile reminded them of a small bar of soap. The NMJL eventually adopted the name “Soap” into its acceptable naming of the White Dragon tile and today both “Soap” and “White” are acceptable names for this tile.
Source: MJME page 5 & page 6 (2020 publication)