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

spamdex

    (Presumably from "spam", "index") word
   spamming.

   "Spamdexing has come a long way from the halcyon days of the
   summer of 1995.  Back then, all one needed to do was add the
   word 'sex' a thousand times at the end of a Web page to
   attract attention from the likes of Lycos.  The search-engine
   operators caught on fast" -- Andrew Leonard, Hotwired 1996
   (http://packet.com/packet/leonard/96/32/index3a.html).

   (1997-04-09)