The manure was shipped dried, because in dry form it weighed a lot less than when wet. But once water (at sea) hit it, not only did it become heavier, but the process of fermentation began of which, of course, 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 as soon as someone went below at night with a lantern, BOOOOM!
Several ships were destroyed in this manner before it was determined what was happening.
After that, the bundles of manure were always stamped with the instruction - 'Stow high in transit' on them, so that the sailors knew to stow it high enough above the lower decks so that any water that came into the hold would not touch this volatile cargo and start the production of methane.
Thus evolved the term -'S.H.I.T' -(Stow High In Transit) which has come down through the centuries and is still in use to this very day.
You probably did not know the true history of this word.
Neither did I.
I had always thought it was a term that described the place you had to go through to get from Wales to Scotland.
Contribution from Jem