Etymology
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pig (n.1)

Middle English pigge "a young pig" (mid-13c., late 12c. as a surname), probably from Old English *picg, found in compounds, but, like dog, its further etymology unknown. The older general word for adults was swine, if female, sow, if male, boar. Apparently related to Low German bigge, Dutch big ("but the phonology is difficult" -- OED).

By early 14c. pig was used of a swine or hog regardless of age or sex. Applied to persons, usually in contempt, since 1540s; the derogatory meaning "police officer" has been in underworld slang at least since 1811.

The pigs frisked my panney, and nailed my screws; the officers searched my house, and seized my picklock keys. ["Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence," London, 1811]

Another Old English word for the animal was fearh, which is related to furh "furrow," from PIE *perk- "dig, furrow" (source also of Latin porcus "pig," see pork). "This reflects a widespread IE tendency to name animals from typical attributes or activities" [Lass].

Synonyms grunter, porker are from sailors' and fishermen's euphemistic avoidance of uttering the word pig at sea, a superstition perhaps based on the fate of the Gadarene swine, who drowned. The image of a pig in a poke is attested from late 14c. (see poke (n.1)). Flying pigs as a type of something unreal is from 1610s.

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pig (v.)

mid-15c., piggen, of sows, "to farrow, to bring forth piglets," from pig (n.1). By 1670s as "to huddle together in a dirty or disorderly manner, as pigs do, hence, generally, "to act or live like a pig" in any sense. Related: Pigged; pigging. Colloquial pig out "eat voraciously" is attested by 1979.

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pig (n.2)

"oblong piece of metal," 1580s, from pig (n.1) on the notion of "large mass."  In Middle English sow also was used of a mass or bar of lead (mid-15c.).

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pig iron (n.)

"iron in pigs," as it comes from a blast furnace, iron that has been run while molten into a mold in sand, 1660s; see pig (n.2) + iron (n.).

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pig-sticker (n.)

also pigsticker, "sharp knife, bayonet," by 1890, from pig (n.1) + agent noun from stick (v.).

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pig-headed (adj.)

also pigheaded; 1610s, "having a head resembling a pig;" 1788 as "stupid and obstinate, unreasonably set in mind;" see pig (n.1) + -headed. Usually, but not always, figurative.

A pig-headed man must be one, who, like a driven pig, always will do exactly the opposite to what other people--in the case of the pig his luckless driver--wish him to do, that is to say he is an obstinate man. [The Sedberghian, June 1882]
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guinea pig (n.)
rodent native to South America, 1660s. It does not come from Guinea and has nothing to do with the pig. Perhaps so called either because it was brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein], or from its resemblance to the young of the Guinea-hog "river pig" [OED], or from confusion of Guinea with the South American region of Guyana (but OED is against this). Pig probably for its grunting noises. In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in medical experiments (by 1865).
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piggy (adj.)

"resembling a pig," 1841, from pig (n.1) + -y (2).

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piglet (n.)

"a small or young pig," 1883, from pig (n.1) + diminutive suffix -let. Earlier name for baby pig was farrow.

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pigpen (n.)

"a pen for pigs," by 1833, from pig (n.1) + pen (n.2).

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