What's the etymology of spam when talking about bulk unsolicited messages?

SPAM is a proprietary name registered by Geo. A. Hormel & Co. in U.S., 1937; probably a conflation of spiced ham. Soon extended to other kinds of canned meat.

In the sense of "Internet junk mail" it was coined by Usenet users after March 31, 1993, when Usenet administrator Richard Depew inadvertently posted the same message 200 times to a discussion group. The term had been used in online text games, and ultimately it is from a 1970 sketch on the British TV show "Monty Python's Flying Circus" wherein a reading of a restaurant's menu devolves into a humorous song consisting (almost exclusively) of the word 'spam.'

How the Word “Spam” Came to Mean “Junk Message

Etymonline traces it to a usenet post as described in Josh61's answer. The post in question was by Joel Furr and reads:

ARMM: n. A USENET posting robot created by Dick Depew of Munroe Falls, Ohio. Originally intended to serve as a means of controlling posts through anon servers (see also {anon servers}). Transformed by programming ineptitude into a monster of Frankenstein proportions, it broke loose on the night of March 31, 1993 and proceeded to spam news.admin.policy with something on the order of 200 messages in which it attempted, and failed, to cancel its own messages. This produced a recursive chain of messages each of which tacked on:

I found this page which offers some (unsourced) alternatives:

• First documented case among Usenet users was March 31, 1993. This is often incorrectly stated to be the first usage of the term spam as referring to spam messages. This first Usenet case came when Richard Depew, who had been playing with some moderation software, accidentally ended up posting around 200 duplicate messages in a row to news.admin.policy newsgroup. The first person to call this spam is thought to be Joel Furr on March 31, 1993. Depew himself when he apologized referred to his messages as spam.

• A more likely “first use” of the word spam, referring to certain electronic messages, comes from MUDs (multi-user-dungeons). This was a sort of real time multi-person shared environment; a somewhat primitive version of The Sims Online or Second Life and the like. In it, users could chat and interact with other people, locations, and objects, as well as create objects and share them with the community. Basically a really advanced chat room. The name MUD comes from the fact that it reminded people of certain aspects of Dungeons and Dragons. In any event, spamming was used here to refer to a few different things including: flooding the computer with random data; “spam the database” by flooding it with new objects; and flooding a chat session with a ton of unwanted text. Basically, anything that had to do with filling other member’s accounts with unwanted electronic junk. One of the earliest documented uses of the word spam from MUDders comes from 1990 when they were, ironically enough, discussing the origins of the word “spam” as referring to electronic junk messages. Undocumented sources say it had been around quite a bit before that among MUDders, which is evidenced by the content of the documented message.

• Others say that the term originated on Bitnet’s Relay, which was a very early chat system in the 1980s. Supposedly, users would occasionally come on and annoy other users with unwanted text, including the actual SPAM SPAM SPAM song from Monty Python.

• Another similar chat system TRS-80 also reported the same phenomenon and also called it spam. Both these latter two chat system origins are not documented, but numerous former users of these systems have stated they remember this term being use commonly among users of these systems.

Some more reflections on the origin of the term can be found on Brad Templeton's blog, here. They basically reiterate the above but are actually sourced.