Old English scitte "purging, diarrhea," from source of shit (v.). Sense of "excrement" dates from 1580s (Old English had scytel, Middle English shitel for "dung, excrement;" the usual 14c. noun seems to have been turd). Use for "obnoxious person" is at least since 1508; meaning "misfortune, trouble" is attested from 1937. Shit-faced "drunk" is 1960s student slang; shit list is from 1942.
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). To not give a shit "not care" is from 1922. Pessimistic expression same shit different day attested by 1989. Shitload (also shit-load) for "a great many" is by 1970. Shitticism is Robert Frost's word for scatological writing. Piece of shit "contemptible person" is by 1916.
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]
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.]
"quadruped of the genus Canis," Old English docga, a late, rare word, used in at least one Middle English source in reference specifically to a powerful breed of canine; other early Middle English uses tend to be depreciatory or abusive. Its origin remains one of the great mysteries of English etymology.
The word forced out Old English hund (the general Germanic and Indo-European word, from root from PIE root *kwon-) by 16c. and subsequently was picked up in many continental languages (French dogue (16c.), Danish dogge, German Dogge (16c.)). The common Spanish word for "dog," perro, also is a mystery word of unknown origin, perhaps from Iberian. A group of Slavic "dog" words (Old Church Slavonic pisu, Polish pies, Serbo-Croatian pas) likewise is of unknown origin.
In reference to persons, by c. 1200 in abuse or contempt as "a mean, worthless fellow, currish, sneaking scoundrel." Playfully abusive sense of "rakish man," especially if young, "a sport, a gallant" is from 1610s. Slang meaning "ugly woman" is from 1930s; that of "sexually aggressive man" is from 1950s.
Many expressions — a dog's life (c. 1600), go to the dogs (1610s), dog-cheap (1520s), etc. — reflect the earlier hard use of the animals as hunting accessories, not pets. In ancient times, "the dog" was the worst throw in dice (attested in Greek, Latin, and Sanskrit, where the word for "the lucky player" was literally "the dog-killer"), which plausibly explains the Greek word for "danger," kindynos, which appears to be "play the dog" (but Beekes is against this).
Notwithstanding, as a dog hath a day, so may I perchance have time to declare it in deeds. [Princess Elizabeth, 1550]
Meaning "something poor or mediocre, a failure" is by 1936 in U.S. slang. From late 14c. as the name for a heavy metal clamp of some kind. Dog's age "a long time" is by 1836. Adjectival phrase dog-eat-dog "ruthlessly competitive" is by 1850s. Phrase put on the dog "get dressed up" (1934) may be from comparison of dog collars to the stiff stand-up shirt collars that in the 1890s were the height of male fashion (and were known as dog-collars from at least 1883).
And Caesar's spirit, ranging for revenge,
With Ate by his side come hot from Hell,
Shall in these confines with a monarch's voice
Cry Havoc! and let slip the dogs of war;
[Shakespeare, "Julius Caesar"]
also dogleg, "bent like a dog's hind leg," 1843, earlier dog-legged (1703), which was used originally of a type of staircase which has no well hole and consists of two flights with or without winders. See dog (n.) + leg (n.).