In the 16th and 17th centuries, everything had to be transported by ship.
Commercial fertilizer had not yet been invented, so large shipments of manure
were common.
It was shipped dry, because in dry form it weighed a lot less. But when water (at sea) hit it, it not only became heavier, the process of fermentation began. A byproduct of fermentation is methane gas. You can probably figure out what happened next.
Methane began to build up below decks and the first time someone came below at night
with a lantern, BOOOOM!
Several ships were destroyed in this manner before it was determined just what was
happening
After that, the bundles of manure were always stamped with the term ' Ship High In Transit ' on them, which meant for the sailors to stow it high enough off 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 ' , (Ship High In Transit) which has come down through the centuries and is in use to this very day.
Thanks for the info, 
