I can't believe the number of searches for "What does spam stand for" I'm getting to my Spam Filtering's Last Stand piece. Hopefully this entry will override that one once this is indexed.
Apparently Google has nominated me to answer this question, so: "Spam" doesn't stand for anything. It's not an acronym. SPAMTM is the name of a meat product by Hormel. Spam came to mean "junk email" by analogy to a Monty Python's Flying Circus skit, wherein a restaurant's good food is drowned out by the spam, much like good content is drowned by spam on the Internet.
The meat product is tolerably good for the price. Internet spam is a public menance. Hormel has been quite good spirited about the whole thing and asks that you not confuse the two, but that you do distinguish between SPAM (all-caps) the meat product and spam (lowercase) the Internet scourge.