Lazy, drawly way, probably American (!) of saying Saint Nicholas. I assume it began around the time of the Clement Clark Moore poem The night before Christmas. Written in 1822.

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American, yes, but not such a lazy or drawly diminutive. Just a simple Anglicized (or Americanized) step from the Dutch "Sante Klaas" / "Sinterklaas", noted as early as 1773.

http://www.etymonline.com/index.php?term=Santa+Claus


Stephen.