Most people have some vague awareness that it came from at first from the spam skit by Monty Python's Flying Circus. In the sketch, a restaurant serves all its food with lots of spam, and the waitress repeats the word several times in describing how much spam is in the items. When she does this, a group of Vikings (don't ask) in the corner start a song:
"Spam, spam, spam, spam, spam, spam, spam, spam, lovely spam! Wonderful spam!"
Until told to shut up.
Thus the meaning of the term at least: something that keeps repeating and repeating to great annoyance. How did the two get connected?
The term got really popular in April of 1994, when two lawyers from Phoenix named Canter and Siegel posted a message advertising their fairly useless services in an upcoming U.S. "green card" lottery. This wasn't the first such abusive posting, nor the first mass posting to be called a spam, but it was the first deliberate mass posting to get that name.
The "green card" spam didn't coin the term, and it wasn't even the first "spam" -- though it was the first really large commercial one.
My research shows the term goes back to the late 1980s and the "MUD" community.
A MUD is a multi-user-dungeon. That's a somewhat archaic term for a real time multi-person shared environment, which is to say a shared world where users can chat, move around and interact with locations and objects in the environment. MUDs were named that because the first reminded people of "adventure" or "Dungeons and Dragons" games that involved jointly exploring a cave or dungeon.
But most people used MUDs to chat, and to play around and impress one another with objects they created. They were at first a highly evolved successor for the chat room.
The term spamming got used to apply to a few different behaviours. One was to flood the computer with too much data to crash it. Another was to "spam the database" by having a program create a huge number of objects, rather then creating them by hand. And the term was sometimes used to mean simply flooding a chat session with a bunch of text inserted by a program (commonly called a "bot" today) or just by inserting a file instead of your own real time typing output.
There are unconfirmed reports as well that the term migrated to MUDs from early "chat" systems. Rich Frueh believes the term originated on Bitnet's Relay, the early chat system that IRC was named after. When the ability to input a whole file to the chat system was implemented, people would annoy others by dumping the words to the Monty Python Spam Song. Peter da Silva reports use in early 80s chat on TRS-80 based BBSs, but feels since they imported other Bitnet Relay customs, the term may have come from there. Another unconfirmed report from a BBS user claims to have seen it defined as a "Single Post to All Messagebases" though this origin seems unlikely in my personal opinion.
Another report describes indirectly a person simply typing "spam, spam..." in a MUD with a keyboard macro until being thrown off around 1985.
