In the 16th and 17th centuries, everything had to be transported by ship and it was also before commercial fertilizer’s’ invention, so large shipments of manure were common. It was shipped dry, because in dry form it weighed a lot less than when wet, but once water (at sea) hit it, it not only became heavier, but the process of fermentation began again, of which a by product is methane gas.
As the stuff was stored below decks in bundles you can see what could (and did) happen. Methane began to build up below decks and the first time someone came below at night with a lantern and boom!
Several ships were destroyed in this way before it was determined jus’ what was happening. After that, the bundles of manure were always stamped with the term ‘Ship High In Transit’ on them, which meant for sailors to stow it high enough off the lower decks that any water that came into the hold would not touch the cargo
Thus evolved the term ‘ S.H.I.T,’ which has come down through the centuries and is in use to this very day. You probably did not know the true history of this word.
Neither did I — as I always thought it was a cowboy term.