As they say you learn something new every day.......

If this is true you learn a lot

Manure 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, 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 t his volatile cargo and start the production of
methane.

Thus evolved the term "S.H.I.T ", (Ship High In Transport) which has come
down through the centuries and is in use to this very day.