https://en.wikipedia.org/wiki/HTTP_referer
Etymology
The misspelling of referrer was introduced in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the "Referer" header field into the HTTP specification.[7][8] The misspelling was set in stone by the time (May 1996) of its incorporation into the Request for Comments standards document RFC 1945[9] (which 'reflects common usage of the protocol referred to as "HTTP/1.0"' at that time); document co-author Roy Fielding remarked in March 1995 that "neither one (referer or referrer) is understood by" the standard Unix spell checker of the period.[10] "Referer" has since become a widely used spelling in the industry when discussing HTTP referrers; usage of the misspelling is not universal, though, as the correct spelling "referrer" is used in some web specifications such as the Referrer-Policy HTTP header or the Document Object Model.[3]