I am talking about the food SPAM! Not the SPAM email! Oh wait, but why they would have the same name?
On 5th July 1937, the canned meat producer Jay Hormel created a product named "Hormel Spiced Ham". Unfortunately the sales is not too good, therefore Jay Hormel organized a naming competition, and the name 'SPAM' won. People generally believed 'SPAM' stands for 'SPiced hAM' or 'Shoulders of Pork and hAM'.
Then why would SPAM become the term for 'garbage emails'? In a sarcastic drama from the show "Monty Python", a couple went for dining and the wife didn't want to have SPAM as dinner. However there were so many people in the restaurant praising the tastiness of SPAM for more than a hundred times, which made the other customers annoyed. Besides, SPAM itself is not a healthy food and does not contain lots of nutrients. That's why it slowly became the name of unsolicited emails.
Thanks for my younger brother @helloworld123 's idea :)