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”).
-  Danskernes Navne, based on CPR data: 8 117 males with the given name Kai (compared to 31 574 named Kaj) have been registered in Denmark between about 1890 (=the population alive in 1967) and January 2005, with the frequency peak in the 1920s. Accessed on 19 June 2011.
- son of Kai: Kaisson
- daughter Kai: Kaisdóttir
Taken into regular use in the 20th century; either borrowed from Danish Kaj, or from a West Frisian baby talk form of Kaimbe , Kempe (fighter, warrior), Gerrit ( =Gerard), Cornelis (= Cornelius) and Kleis ( =Nicholas).
Kai m, (f)
kai (“sea, sea water”) ; also a short form of compound given names containing this word.
- Hawaii State Archives: Marriage records Kai occurs in 19th century marriage records as the only name (mononym) of 10 women and 13 men.
- Social Security Administration: Popular Baby Names by State: Kai was included in the top hundred first names for boys born in the State of Hawaii in 1995-2008.