Continuing the discussion from How to combat manspreading on the subway - sit on the offender:
To the best of my knowledge, the origin of the phrase was in the Army. A spade is officially known as a “personal entrenching tool”, and many other common things also have overly complex names. That’s why they love acronyms so much. As a veteran and a former POW, I would be more inclined to think Mr. Vonnegut picked it up from that context.
But this gets me to thinking: how many other phrases come from similar innocuous roots, but due to words evolving we assume more nasty origins?