There's a lot of false information out there about why we refer to mass unsolicited commercial mailing as spam. This article sifts through the falsehoods to reveal the truth.
Some people will try and tell you that spam is an acronym. Shit Posing As Mail and Stupid Boring Annoying Messages being two of the most popular. This is not the origin or the word spam as it applies to unsolicited email.
There is also a story about downloading(usually pictures of naked woman, though sometimes performance cars) that, upon completing the download turned out to be a picture of a can of spam. The word spam is then said to be posted as a warning below these messages, as signifying a waste of time and effort.
Not surprisingly the most common story is the true one. The word spam was used in a famous Monty Python sketch involving a couple in a shop attempting to order a meal. All items in the shop came with spam, usually three to four helpings. During the entire sketch the a group of vikings starts singing the refrain "Spam, spam, spam, spam...lovely spam...wonderful spam". The song gets progressively louder during the sketch, eventually drowning out the conversation - much as spam mail does today.
The term spam made the jump to the electronic world through chatroom spam. In the early 1980's in People Link chatrooms it developed as a way to see off unwanted members from a forum of friends having a conversation. When a new member tried to hijack the conversation, the existing members of the room would begin typing the lines from the Monty Python sketch referred to above. The remaining members would sit back and laugh, or get a drink, but offer no further comment until the unwanted member left. being 'spammed out of a room' quickly made the jump to other online mediums, most notably MUDS where any unwanted communication was referred to as spamming and the first rules against SPAM were developed as early as 1981 'No Spamming Public Channels'.
Today, spam is used in almost every arena to describe unwanted and annoying messages.
