(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.