The word "spam" originally referred only to one of the brands of Hornel Foods Corporation, specifically to a kind of canned meat.

Following a 1970 sketch in Monty Python's Flying Circus, the term was adopted in the 90's as a way to refer to unsolicited bulk mail, and is part of the general slang all around the world.

If you see that sketch you'll probably understand why this term was adopted:

:-)