SPAM is the trademarked name of a canned meat product (“spicy meat and ham”) that was first produced by Hormel Foods Corp. in Austin, Minn., in 1937 and became widely known from its use by U.S. Armed Forces during World War II.
The use of the term to mean unwanted mass e-mail derives from a famous Monty Python sketch, first broadcast in 1970, where a couple ordering breakfast is confronted with a menu that is heavy on one specific ingredient, as you can see in this video.
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 bacon spam tomato and spam …
Chorus of Vikings (chanting): Spam spam spam spam …
Waitress: … spam spam spam egg and spam; spam spam spam spam spam spam baked beans spam spam spam …
Vikings (singing): Spam! Lovely spam! Lovely spam!
The wife doesn’t want spam, but she has little choice.
Pretty well sums up most people’s attitude toward unwanted commercial e-mail.