Meaning:
What a great idea (often expressed in a somewhat sarcastic way)
Background:
In 1928, Otto Frederick Rohwedder of Davenport, Iowa produced the first commercially viable bread slicing machine. It was first put into use by the Chillicothe Baking Company in Missouri who advertised the resulting product as "the greatest forward step in the baking industry since bread was wrapped". It is believed that this slogan provided the genesis of the "the greatest thing since sliced bread" - which became a popular phrase around the middle of the 20th century.
As an interesting aside, in 1943, there was a shortlived ban on sliced bread as a wartime conservation measure. It's not entirely clear what was behind the ban but it seems to have been generally related to shortages in materials - one source quotes a steel shortage being the issue, while another says that the ban came into being mainly over concerns that sliced bread required heavier wrapping than unsliced in order to keep it from drying out. Apparently, the ban was also intended to counteract an increase in the price of flour - presumably the logic being that bakers could charge a small premium for sliced bread so a lower price for a whole loaf would somewhat offset the rise in flour prices.
Perhaps the ban was also intended to lower the consumption of bread itself. Interestingly, the introduction of sliced bread led to an increase in bread consumption - apparently the uniform, thinner slices (than most people cut themselves) and the ease of reaching for another slice had led to an increase in the amount of bread eaten. The knock on effects of the humble bread slicer did not end there - not only did the increase in bread consumption have a positive affect on the sales of jams and spreads, it also increased the popularity of soft loaves such as Wonderbread, as these were relatively difficult to cut by hand.
The ban on sliced bread lasted less than two months. The official reason given by Food Administrator Claude R. Wickard was that "the savings are not as much as we expected, and the War Production Board tells us that sufficient wax paper to wrap sliced bread for four months is in the hands of paper processor and the baking industry."
The unofficial reason might have had more to do with consumer backlash as illustrated by the letter below, penned by an irate housewife and published in the New York Times in late January 1943:
"I should like to let you know how important sliced bread is to the morale and saneness of a household. My husband and four children are all in a rush during and after breakfast. Without ready-sliced bread I must do the slicing for toast - two pieces for each one - that's ten. For their lunches I must cut by hand at least twenty slices, for two sandwiches apiece. Afterward I make my own toast. Twenty-two slices of bread to be cut in a hurry!"
