Long before "spam" meant unwanted email, it meant canned meat — a real product, Hormel's SPAM, that has been sold since 1937. The internet didn't invent the word. It borrowed it, by way of a joke that had already been circulating in geek culture for years before junk email became the epidemic it is today.
Where the word actually comes from
The word's path to meaning "unwanted repetitive garbage" runs through a 1970 Monty Python sketch set in a café where nearly every item on the menu contains Spam, and a group of Vikings in the corner keep loudly singing the word "Spam" over and over, drowning out the other characters' conversation entirely. That image — something repeated so much it crowds out everything else — was exactly the metaphor that early online communities reached for once people started flooding chatrooms, multiplayer game spaces, and Usenet discussion groups with repetitive junk text in the late 1980s and early 1990s. Calling it "spam" was originally an in-joke, referencing a sketch that regulars on early networks would have known well.
From there it spread. By the early-to-mid 1990s, "spamming" was established slang across Usenet and early chat culture for flooding a space with unwanted repeated messages, whether that meant a prank, an argument tactic, or advertising.
The email that (arguably) started it
The specific act of unsolicited bulk email predates the word being applied to it. In May 1978, a marketer at Digital Equipment Corporation named Gary Thuerk sent a message to several hundred users on ARPANET — the government-and-university research network that predated the modern internet — advertising a product demonstration for a new line of DEC computers. It reached only a few hundred people, since ARPANET itself only had a few thousand users total at the time, but it annoyed a lot of them enough that it's now widely remembered as the first piece of unsolicited commercial bulk email, sent well over a decade before the word "spam" was ever applied to the practice.
The Green Card notice
The moment usually credited with cementing "spam" as the term for junk commercial messaging specifically came in 1994, when two Arizona immigration lawyers, Laurence Canter and Martha Siegel, posted an advertisement for their green card lottery legal services to thousands of Usenet newsgroups simultaneously, regardless of whether the groups had anything to do with immigration. It was one of the first times a commercial message had been blasted indiscriminately across the entire network rather than sent to an interested audience, and the backlash was immediate and furious — Usenet users flooded the lawyers' inbox and phone line in retaliation, and the incident became a widely publicized case study in exactly what "spam" meant and why people hated it. Canter and Siegel, unrepentant, later co-wrote a book essentially arguing spam was the future of advertising.
Why it never went away
Unsolicited bulk messaging turned out to be too cheap to resist. Sending a million emails costs almost nothing compared to a million physical letters, and even a tiny response rate can be profitable. That basic economic fact — not any particular villain — is why spam scaled from an annoying newsgroup post into an entire adjacent industry of filters, blocklists, and legislation. Today, automated filtering catches the overwhelming majority of it before it ever reaches an inbox, which is easy to forget precisely because the filtering works. The word stuck around from a decades-old sketch about a diner that couldn't stop saying its own name — which, it turns out, was a pretty good description of what junk email would go on to do to everyone's inbox.