The term 'store high in transit' refers to what?

If you can believe it, it comes from the story where in the 17th century, bales of animal manure were dried and transported by ship. They often got wet during transit and over a period of time gave off methane gas. Any hapless crew who walked into the hold with a naked flame met with a nasty surprise. Thus began the alleged tradition of S.H.I.T. being stamped on the bales, meaning "store high in transit" so that the bales did not get wet whilst in the ships holds.

That story is an urban legend.

In 2002, an alleged acronymic origin for shit appeared on the Internet. According to this tale, the word is from an acronym for Ship High In Transit, referring to barges carrying manure. This is a complete fabrication and absurd on its face. All it takes to disprove it is to look up the word in any decent dictionary. Remember, anytime someone posits an acronymic word origin, chances are that it is utterly false.