The term Shit came from the old days of shipping. They used to ship cross-country on trains, or internationally on boats, all kinds of cargo, all mixed together. They used to ship cow manure, for fertilization processes. As you know, this material emits flammable fumes, called methane. When the manure was packed in closed quarters, the fumes would build up, and one way or another, would ignite, either from the coal powered engines, or another form of combustion. They weren't going to just stop the shipment of manure all together, so to eliminate the build up of the fumes, they would store the manure up higher on the freight liners, and boats, so the fumes wouldnt build up (because fumes only rise). this made them label all manure-type products: Ship High in Transit. or SHIT
I dont know if you belive me, but, that is the explanation I was given.