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. The notion that it is a recent word might be partly because it was taboo from c. 1600 and rarely appeared in print (neither Shakespeare nor the KJV has it), and even in "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").
Extensive slang usage; meaning "to lie, to tease" is from 1934; that of "to disrespect" is from 1903. Shite, now a jocular or slightly euphemistic and chiefly British variant of the noun, formerly a dialectal variant, reflects the vowel in the Old English verb (compare German scheissen); the modern verb has been influenced by the noun. 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.]