Almost everyone knows what Spam is and almost no one can say what the name stands for.
Hormel introduced the canned product on July 5, 1937, according to Wikipedia. That year Ken Daigneau, the brother of a Hormel executive, won a contest to name it and received $100. Etymonline says Geo. A. Hormel and Company registered Spam as a trademark in 1937 and calls it "probably a conflation of spiced ham."
That is the popular answer, and it is probably right. It is also not confirmed. Wikipedia quotes Hormel saying the meaning "is known by only a small circle of former Hormel Foods executives." Mental Floss reports that Hormel's official explanation is "spiced ham," and that the company has also said it stands for "shoulder of pork and ham." Wikipedia treats the second as speculation. So two company-linked readings exist, and the company has not settled it. Hormel's website offers a playful backronym for sizzle, pork and mmm, which is a joke rather than an origin.
The email sense came later and by an odd route. In 1970 the Monty Python's Flying Circus sketch set in a restaurant repeated "Spam" until it drowned out all other conversation. Mental Floss says players of early multi-user dungeon games were the first to link that repetition to repetitive text, and that users flooded chats by dumping the song into them.
Usenet carried the word further after that. Etymonline says the junk-mail sense was coined sometime after 31 March 1993, when administrator Richard Depew accidentally posted one message about 200 times. It gives no exact first date for the sense. Wikipedia adds that by the 1990s the name had been adopted for unsolicited electronic messages.
Etymonline also records "spam" for something bland or unexciting by 1959. The meat is the only sense of the word that has an owner, and the trademark has not stopped the lowercase one.