I read this recently, but cant vouch fot the truth thereoff.
Untill the coming of the railways most Westcountry goods traveled by sea. One of the most common cargoes was various forms of manure, Seaweed, sand and shells and dung. these were all dried out for transport to save weight
and loaded in the ships hold. Sometimes water would leak in with disasterous results, the cargo would swell up unbalancing the ship and give off methane resulting in explosions. To try and avoid this containers were marked Ship High In Transport, leading to the contents becoming known as SHIT.