I can answer that ... well the internet version anyway:

"The history of calling inappropriate postings in great numbers 'Spam' is from a Monty Python skit where a couple go into a restaurant, and the wife tries to get something other than Spam. In the background are a bunch of Vikings that sing the praises of Spam. Pretty soon the only thing you can hear in the skit is the word 'Spam'. That same idea would happen to the Internet if large scale inappropriate postings were allowed. You couldn't pick the real postings out from the Spam."