. The noun is O.E. scitte "purging;" sense of "excrement" dates from 1585, from the verb. Despite what you read in an e-mail, "shit" is not an acronym.
The notion that it is a recent word may be because the word was taboo from c.1600 and rarely appeared in print (neither Shakespeare not 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; verb meaning "to lie, to tease" is from 1934; that of "to disrespect" is from 1903. Noun use for "obnoxious person" is since at least 1508; meaning "misfortune, trouble" is attested from 1937. Shat is a humorous past tense form, not etymological, first recorded 18c. Shite, now a jocular or slightly euphemistic variant, formerly a dialectal variant, reflects the vowel in the O.E. verb (cf. Ger. scheissen). Shit-faced "drunk" is 1960s student slang; shit list is from 1942. To not give a shit "not care" is from 1922; up shit creek "in trouble" is from 1937. Scared shitless first recorded 1936.
"The expression [the shit hits the fan] 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]