Pig is native, from an Old English word that only survives in compounds, and — like dog — its origin beyond that is simply unknown. That is unusual and worth noting: the everyday animal vocabulary of English is mostly transparent all the way back, but pig and dog are both blanks.
It is the native half of the pig-and-pork pair, alongside cow and beef, sheep and mutton, calf and veal. In Old English the general word for the adult animal was swine, with sow and boar for the sexes; pigge meant a young one, and only in the early 1300s did it spread to the animal at any age or sex. Applied to people it has been an insult since the 1540s, and the sense of a police officer is older than most people assume — it appears in a London slang dictionary of 1811.
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
(1640s),
porker
(1650s) 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.
also from mid-13c.
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.
also from mid-15c.
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.).
also from 1580s