It seems that in days of yore, manure was shipped on boats. It was allowed to dry to reduce weight. In the hold, however, it could get wet and start to ferment. This produced methane gas and if a hapless sailor were to approach with a lit torch, the results could be disasterous.

So the bales of manure were labeled "ship high in transit" to mean to be loaded on the boat high enough to avoid water contamination. Or if they they did get wet that the methane gas would escape harmlessly.

The note was reduced to an acronym and hence the origin of the present English word.

Glenda