A meat product from Hormel Foods. Officially SPiced hAM, although nobody really knows for sure. For all we know it might be Squirrels, Possums And Mice.
SPAM was mentioned in a sketch by MontyPythonsFlyingCircus, where a customer attempting to order a dish -without- spam in it is drowned out by a number of noisy vikings singing "SPAM SPAM SPAM.." loudly in the background. When Usenet started to get flooded with annoying advertising (I wonder how many people here can remember back when usenet and email suffered NO advertising at all!) the comparison was obvious and the annoying advertising quickly became knows as "Spam".
Hormel foods have asked (fairly nicely) that we please refer to EMP/UCE/UBE as "spam" in lowercase, and not use their trademark square can image any more.