As described in historical dictionaries

An Etymological Dictionary of Family and Christian Names (1857)

(origin: Dutch.) The son of Klaas, the abbreviation of Nicholas among the Dutch. Klaas-son, the son of Nicholas. So Santa Klaas for St. Nicholas.

Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.


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