The term was originally applied to annoying messages on the Usenet netnews network and then made its way into the lexicon of the e-mail world. There's nothing at all funny about e-mail spam. It's difficult to overstate the waste of money, time, and good will triggered by the uncaring organizations and individuals who generate this trash.
Spammers compile the e-mail addresses for their massive mailings from many varied sources.
It is unfortunately almost impossible for the average e-mail recipient to accurately determine the origin for much of the spam that they receive. Spam messages often have forged sender addresses and falsified message headers, and are frequently relayed in vast quantities through naive or careless third party sites, exploiting openings in the mail server software systems that were originally designed for a much more benign and cooperative networking environment. The spammers frequently attempt to justify their abusive practices on First Amendment grounds, or by pointing to physical bulk mail advertising delivered by the postal service as a comparable situation. Most reputable firms have quickly learned that sending out unsolicited commercial e-mail will result in many angry recipients who typically will have no reservations about making their displeasure known. If recipients feel that they never really opted-in for such mailings, or that they have no simple way to control them, they are likely to consider such messages to be simple spam, to be complained about and possibly reported to their ISPs just like all the rest.