Spam

Electronic junk mail or junk newsgroup postings. Some people define
spam even more generally as any unsolicited e-mail. However, if a
long-lost brother finds your e-mail address and sends you a message,
this could hardly be called spam, even though it's unsolicited.
Real spam is generally e-mail advertising for some product sent
to a mailing list or newsgroup.
In addition to wasting people's time with unwanted e-mail, spam
also eats up a lot of network bandwidth. Consequently, there are
many organizations, as well as individuals, who have taken it upon
themselves to fight spam with a variety of techniques. But because
the Internet is public, there is really little that can be done
to prevent spam, just as it is impossible to prevent junk mail.
However, some online services have instituted policies to prevent
spammers from spamming their subscribers.

There is some debate about the source of the term, but the generally
accepted version is that it comes from the Monty Python song,
"Spam spam spam spam, spam spam spam spam, lovely spam, wonderful spam…"
Like the song, spam is an endless repetition of worthless text.
Another school of thought maintains that it comes from the
computer group lab at the University of Southern California
who gave it the name because it has many of the same characteristics
as the lunchmeat Spam:

Nobody wants it or ever asks for it.
No one ever eats it; it is the first item to be pushed to the
side when eating the entree.

Sometimes it is actually tasty, like 1% of junk mail that is
really useful to some people.