It seems like the two spellings were used interchangeably until about 1960: https://books.google.com/ngrams/graph?content=referer%2C+referrer&year_start=1800&y

It seems like the two spellings were used interchangeably until about 1960: https://books.google.com/ngrams/graph?content=referer%2C+referrer&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Creferer%3B%2Cc0%3B.t1%3B%2Creferrer%3B%2Cc0 https://books.google.com/ngrams/graph?content=referer%2C+ref... Edit: Actually, that doesn't seem to be true -- although there's some usage of "referer" in English, most of the hits in Google Books prior to 1960 turn out to be for the Old French word "referer".