出典:Wiktionary
Taken into regular use in the 1990s, with earlier popularity peaks in Scandinavia and Germany. The medieval Danish Kaj is possibly of Roman origin, Latinized as Caius, like the rare medieval English male given name Kay. The German Kai may also derive from a West Frisian pet form of Gerard, Cornelius, Nicholas, or Kampe "warrior". In the U.S. Kai has also been explained as Hawaiian kai (“sea”).
Kai