From HSFB:

Just in case you were wondering how the S word evolved:

Back in the 1600's, they transported everything by boat. Since Spain was trying to set up a New World, they were shipping planting necessities over. And, of course, what is planting without manure? But sometimes the manure would get wet and give off methane gas or whatever it is that's flammable. Then some guy would go down there with a torch and~~BOOM. They finally figured out what was happening, so they marked every box of manure with some letters... S----- H----- I----- T standing for ship high in t-something....

Just in case you were wondering :P