Rule 146A, Section B, Subsection 32:  Nomenclature  (also known as the Silly Name Rule)

Enacted February 23, 2018

No player shall be allowed to join a team as an active player whose name is just plain silly, either as a result of an intentional change on the player’s part, or the unfortunate circumstances of when a player of foreign heritage has a name that is perfectly normal in his home country, but is just plain silly in English.

For more information, see Kusuke Fukudome.