Old English scitan, from Proto-Germanic *skit- (source also of North Frisian skitj, Dutch schijten, German scheissen), from PIE root *skei- "to cut, split." The notion is of "separation" from the body (compare Latin excrementum, from excernere "to separate," Old English scearn "dung, muck," from scieran "to cut, shear;" see sharn). It is thus a cousin to science and conscience.
"Shit" is not an acronym. Nor is it a recent word. But it was taboo from c. 1600 and rarely appeared in print (neither Shakespeare nor the KJV has it), and even in the "vulgar" publications of the late 18c. it is disguised by dashes. It drew the wrath of censors as late as 1922 ("Ulysses" and "The Enormous Room"), scandalized magazine subscribers in 1957 (a Hemingway story in Atlantic Monthly) and was omitted from some dictionaries as recently as 1970 ("Webster's New World"). [Rawson]
It has extensive slang usage; the meaning "to lie, to tease" is from 1934; that of "to disrespect" is from 1903. Also see shite. Shat is a humorous past tense form, not etymological, first recorded 18c.
To shit bricks "be very frightened" attested by 1961. The connection between fear and involuntary defecation has generated expressions in English since 14c. (the image also is in Latin), and probably also is behind scared shitless (1936).
Alle þe filþ of his magh ['maw'] salle breste out atte his fondament for drede. ["Cursor Mundi," early 14c.]
Middle English shit "diarrhea," from Old English scitte "purging, diarrhea," from source of shit (v.). The general sense of "excrement" dates from 1580s (Old English had scytel, Middle English shitel for "dung, excrement;" the usual 14c. noun for natural discharges of the bodies of men or beasts seems to have been turd or filth). As an exclamation attested in print by 1920 but certainly older. Use for "obnoxious person" is by 1508; meaning "misfortune, trouble" is attested from 1937.
Shit-faced "drunk" is 1960s student slang; shit list is from 1942. Shit-hole is by 1937 as "rectum," by 1969 in reference to undesirable locations. Shitload (also shit-load) for "a great many" is by 1970. Shitticism is Robert Frost's word for scatological writing.
Up shit creek "in trouble" is by 1868 in a South Carolina context (compare the metaphoric salt river, of which it perhaps a coarse variant). Slang not give a shit "not care" is by 1922. Pessimistic expression same shit different day is attested by 1989. To get (one's) shit together "manage ones affairs" is by 1969. Emphatic shit out of luck is by 1942.
The expression when the shit hits the fan "alluding to a moment of crisis or its disastrous consequences" [OED] is attested by 1967.
The expression is related to, and may well derive from, an old joke. A man in a crowded bar needed to defecate but couldn't find a bathroom, so he went upstairs and used a hole in the floor. Returning, he found everyone had gone except the bartender, who was cowering behind the bar. When the man asked what had happened, the bartender replied, 'Where were you when the shit hit the fan?' [Hugh Rawson, "Wicked Words," 1989]
also shithead, "objectionable or contemptible person," by 1961, from shit (n.) + head (n.). Piece of shit for "contemptible person" is by 1916; shit-sack or shitsack in this sense is noted by 1769, in reference to the time of Charles II, as an "opprobrious appellation by which the Nonconformists were vulgarly distinguished." Simple shit (n.) for "obnoxious person" is by 1510s.
"defiled with excrement," late 14c., past-participle adjective from shit (v.). From 1540s in transferred sense of "very unpleasant."
a colloquial modern alternative spelling of shit (n.), attested by c. 1740. Now a jocular or slightly euphemistic and chiefly British variant of the noun, it formerly was a dialectal variant, and it reflects the vowel in the Old English verb (compare German scheissen). The modern verb in English has been influenced by the noun.