The Collaborative International Dictionary of English v.0.48:

Referrer \Re*fer"rer\ (r?*f?r"r?r), n.
   One who refers.
   [1913 Webster]

The Free On-line Dictionary of Computing (30 December 2018):

referer
referrer

    A misspelling of "referrer" which somehow
   made it into the HTTP standard.  A given web page's
   referer (sic) is the URL of whatever web page contains the
   link that the user followed to the current page.  Most
   browsers pass this information as part of a request.

   (1998-10-19)