Spam originally meant "flooding a [Usenet] newsgroup with irrelevant or inappropriate messages" (The Jargon File). This original sense derived from the overuse of the word "spam" in a sketch performed by Monty Python's Flying Circus. The sketch begins as follows:

Mr. Bun: Morning.

Waitress: Morning.

Mr. Bun: Well, what you got?

Waitress: Well, there's egg and bacon; egg, sausage 
and bacon; egg and spam; egg, bacon and spam; egg, 
bacon, sausage and spam; spam, bacon, sausage and 
spam; spam, egg, spam, spam, bacon and spam; spam, 
sausage, spam, spam, spam, bacon, spam, tomato and 
spam; spam, spam, spam, egg and spam; (Vikings start 
singing in background) spam, spam, spam, spam, spam, 
spam, baked beans, spam, spam, spam and spam...

In recent years, the use of "spam" expanded to include unsolicited commercial e-mail. In case you don't know, spam is a luncheon meat consisting of compressed pork shoulder with a bit of ham tossed in. It is produced by Hormel Foods who even have an Official Spam Home Page.