Nicknames

[J_Robo]

How did Police officers ever get the name Pig?

[Measure_for_Measure]

It stands for “Pride, Integrity, Guts”.
From http://www.wordorigins.org/wordorp.htm

This derogatory term for a police officer is thought by many to be a usage coined in the 1960s. Various explanations for the term have been posited. Among them is the idea the term derives from the gas masks worn by riot police in the 1960s that made the officers look like pigs. Another is that the term is derived from George Orwell’s Animal Farm, where the pigs ran the farm and took away the other animals’ liberties.

Both these, as well as any other twentieth century explanations are incorrect. The term predates the 1960s by at least 150 years. (Also, the dogs were the police officers in Animal Farm not the pigs.)

The OED2 has pig being used as a term for a contemptuous person as early as 1546. The earliest cite for a police officer in particular is from the Lexicon Balatronicum of 1811, which defines pig as “a China Street Pig, a Bow Street officer.” The Bow Street Runners were an early police force of London, named after the street that housed their headquarters. The Lexicon Balatronicum also offers “floor the pig and bolt,” meaning to knock the policeman down and run. According to Partridge, by 1873 the term’s usage was restricted to plain-clothes officers. The term was an underground term, part of the criminal argot, until it emerged into the mainstream in the 1960s.

(OED2 = Oxford English Dictionary)