According to The Internet Society...

" The term "spam" as it is used to denote mass unsolicited mailings or
netnews postings is derived from a Monty Python sketch set in a
movie/tv studio cafeteria. During that sketch, the word "spam" takes
over each item offered on the menu until the entire dialogue consists
of nothing but "spam spam spam spam spam spam and spam." This so
closely resembles what happens when mass unsolicited mail and posts
take over mailing lists and netnews groups that the term has been
pushed into common usage in the Internet community."

My Team Fortress 2 Backpack [optf2.com]