While the GI insurance suggestion sounds reasonable, it must go back to World War I. I have heard this phrase as far back as I remember and I can remember to the 50s pretty well.   I did not get the impression when I heard it that the concept was new as everyone knew what it meant if not where it came from. I would expect origins to go back to early 20th century, but it is interesting that the phrase is not common in the UK and Oz.