It is widely believed the term spam is derived from the 1970 SPAM sketch of the BBC television comedy series "Monty Python's Flying Circus".
The sketch is set in a café where nearly every item on the menu includes SPAM luncheon meat. As the waiter recites the SPAM-filled menu, a chorus of Viking patrons drowns out all conversations with a song repeating "SPAM, SPAM, SPAM, SPAM... lovely SPAM, wonderful SPAM", hence "SPAMming" the dialog. The excessive amount of SPAM mentioned in the sketch is a reference to British rationing during World War II. SPAM was one of the few meat products that was excluded from rationing, and hence was widely available.
There are three popular false etymologies of the word "spam". The first is that "spamming" is what happens when one dumps a can of SPAM luncheon meat into a fan blade. The second is the backronym "shit posing as mail." The third is similar, using "stupid pointless annoying messages."
