Etymology
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turd (n.)

Old English tord "piece of excrement," from Proto-Germanic *turdam (source also of Middle Dutch torde "piece of excrement," Old Norse tord-yfill, Dutch tort-wevel "dung beetle"), from PIE *drtom, past participle of root *der- "to split, flay, peel;" thus "that which is separated ("torn off") from the body" (compare shit (v.) from root meaning "to split;" Greek skatos from root meaning "to cut off; see scatology). As a type of something worthless and vile, it is attested from mid-13c. Meaning "despicable person" is recorded from mid-15c.

A tord ne yeue ic for eu alle ["The Owl and the Nightingale," c. 1250]
Alle thingis ... I deme as toordis, that I wynne Crist. [Wyclif, Philippians iii.8, 1382; KJV has "I count all things ... but dung, that I may win Christ"]
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dog (v.)

"to track as a hunting dog does, keep at the heels of," 1510s, see dog (n.). Related: Dogged; dogging.

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dog (n.)

"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"]
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dog-leg (adj.)

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.).

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dog-gone (adj.)

also doggone, colloquial minced epithet, by 1849, Western American English, a "fantastic perversion of god-damned" [Weekley]. But Mencken favors the theory that it is "a blend form of dog on it; in fact it is still often used with it following. It is thus a brother to the old English phrase, 'a pox upon it,' but is considerably more decorous." Dog on it was the usual early spelling, so it was perhaps at least felt as such by those using it.

But there are many examples of similar words serving as euphemistic perversions of God: Compare dod for "God" in many oaths (late 17c. through 19c.); dodgasted (probably "God-blasted," in use late 19c., early 20c.); dod-rot (1842).

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corn-dog (n.)

"frankfurter dipped in cornmeal batter, fried, and served on a stick," 1949, American English; see corn (n.1) + hot dog. Said to have been introduced by Vaudeville performers Neil and Carl Fletcher in 1942 at the Texas State Fair.

Be the first to serve this delicious new sandwich. No special ingredients needed. Cooks 1 to 4 sandwiches at a time in 5 minutes. Costs only 5 ¢ each to make—sells for 15 ¢ to 20 ¢ apiece. Recipe and instructions shipped with oven. [advertisement for Dixie Corn Dog Ovens in The Billboard, Feb. 26, 1949]
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dog-paddle (n.)

"type of swimming stroke resembling a dog's swimming," 1860, from dog (n.) + paddle

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dog-tired (n.)

"as tired as a dog after a long chase," 1806.

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