In the 16th and 17th centuries, everything was transported by ship. It was also before commercial fertilizer's invention, so large shipments of manure were common.

In dry form it weighed a lot less than when wet, but once water (at sea) hit it, it not only became heavier, the process of fermentation began again, of which a byproduct 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, BOOOOM!

A great many 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 "S.H.I.T" on them which meant to the sailors to "Ship High In Transit."

In other words, 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.

You probably did not know this true history of the word.

Neither did I. I always thought it was a golf term.