Note: Admins, please forgive the use of the term in this post since it is only used here to detail the history and usage of the word.
Shit: Etymology: The word is likely derived from Old English, having the nouns scite (dung, attested only in place names) and scitte (diarrhoea), and the verb scītan (to defecate, attested only in bescītan, to cover with excrement); eventually it morphed into Middle English schītte (excrement), schyt (diarrhoea) and shiten (to defecate), and it is virtually certain that it was used in some form by preliterate Germanic tribes at the time of the Roman Empire. The word may be further traced to Proto-Germanic *skit-, and ultimately to Proto-Indo-European *skheid-. The word has several cognates in modern Germanic languages, such as German Scheiße, Dutch schijt, Swedish skit, Icelandic skítur, Norwegian skitt etc. Ancient Greek had 'skor' (gen. 'skatos' hence 'scato-'), from Proto-Indo-European *sker-, which is likely unrelated.
False etymology: A popular belief is that the word shit originated as an acronym for "Ship High In Transit", referring to the apparent need to stow manure well above the water line when transporting it by ship. This has been shown to be a myth.
Sources:
Harper, Douglas. "shit". Online Etymology Dictionary.
Online Etymology Dictionary
"Shit is not an acronym". Online Etymology Dictionary.
"shit" is not an acronym
Mikkelson, Barbara (8 July 2007). "Etymology of Shit". Snopes.
snopes.com: Etymology of Shit