A good online definition can be found at http://www.msg.net/kadow/answers/s.html#spam:
"Spam: A popular canned meat product. Also, bulk, mass, or repeated posting or mailing of substantially identical messages. The emphasis is on the multiple sending, either many copies to one destination, or one copy to many destinations. This is a reference to the famous Monty Python Spam sketch."

They even have links to the official Spam (meat) site, and the script of the inspirational Python sketch...

Rudi - a great collection of online dictionaries can be found at http://www.onelook.com/