Unwanted bulk message, named after a canned meat, named after nobody is quite sure what.
Documented
From spiced ham to Usenet
Hormel launched SPAM in 1937, plausibly from spiced ham. In 1970 Monty Python's cafe sketch chanted the word until nothing else could be heard. Early 1990s Usenet users adopted it for messages that drowned out everything else.
Assessment: Documented lineage: trademark, sketch, newsgroup. One of the best-attested word journeys in tech.
Stories
The Vikings did it
In the sketch it is singing Vikings who obliterate all conversation with the word. That image is exactly why early internet users reached for it: spam is the thing that repeats until communication dies.
Assessment: Folklore about television, accurate as description. The metaphor chose itself.
Interesting claims
Hormel lost the word and kept the meat
The corporate subplot: Hormel fought trademark cases against anti-spam software names and mostly lost where the use was email-related. A company watched its brand become the synonym for digital garbage.
Assessment: Documented in trademark rulings. Language repossesses what marketing builds, without appeal.
Sources and further reading
Hormel company history, 1937 trademark
Monty Python, the Spam sketch (1970)
Brad Templeton's account of the 1994 Green Card lottery spam
Hormel trademark rulings on email usage
Worth asking
A company spent ninety years building a brand and a comedy sketch renamed the internet's biggest nuisance after it. Who owns a word, really?