The Rice University Neologisms Database

to send out unwanted email to many different persons, with no method of selection in who one is sending them to. Usually takes the form of mass produced advertisements. the word apparently evolved from the product, Spam, a mass produced canned meat product. Presumably, Spam would be an unwanted meal if one were served it, and so the word became expanded to mean unwanted mass produced mail message, though this could be a folk etymology.