Little do many people know, the term actually refers to and originates from the glorious canned luncheon meat of the same name. In the 90s when email became popular, users began referring to a classic Monty Python sketch to talk about these unwanted but ubiquitous messages. In the sketch, a couple is dining at a restaurant in which Spam is part of nearly every item on the menu, much to the chagrin of the wife.
Email users adopted the "spam" moniker to refer to any unwanted, mass emails — because Spam (the meat) was often unwanted and yet everywhere.
Before being the butt of a joke in 1970, though, it was on the losing end of soldiers' jokes during WWII. As we'll see later, Spam supplied our armed forces with many of their meals. Prior to WWII, however, it enjoyed a brief stint as a popular and innovative product born from the ingenuity of southern Minnesotans.