Where did the word s.h.i.t come from?

An unpleasant slang Word of our modern times derives from an old English Word 'scite' or 'scitan' meaning 'dung' and may be of Germanic origin.There is an Old Dutch Word 'schitte' and a Norse Word 'skite' meaning to defecate or excrement . There is also a Latin Word ' excrementum' based on the notion of separation from the body.

Trivia:

There is an urban legend about the term, claiming the term started as an acronym. According to the story, manure was hauled on sailing vessels down in the cargo holds. A problem with that would be when the methane gas built up, and someone would go down there with a lantern. That could cause a fire. So to prevent this, they started attaching a warning to the barrels, saying to "Ship High In Transit," or to ship above deck, in other Words. Then supposedly, that warning got shortened to just four letters, and thus became the common rude Word. However, this is likely just a made up tale, according to Snopes, so it is best to trust the Word etymology above.