Originally and still the canned meatish material, I think it got its new definition via the Monty Python Spam song, with its repetitive "Spam-Spam-Spam-Spam" refrain, which keeps interfering with the characters' conversation in a well known MP skit. It's not an acronym. It is the slang term for e-junk, taken after Hormel's (in)famous canned pork by-product food-like substance.