This:<Troll Alert><Troll Alert><Troll Alert><Troll Alert><Troll Alert><Troll Alert>On the matter of policeman being referred to as "pigs"
If you thought the term pig arose in the 1960s, you're in for a surprise. The OED cites an 1811 reference to a "pig" as a Bow Street Runner--the early police force, named after the location of their headquarters, before Sir Robert Peel and the Metropolitan Police Force (see above.) Before that, the term "pig" had been used as early as the mid-1500s to refer to a person who is heartily disliked.
The usage was probably confined to the criminal classes until the 1960s, when it was taken up by protestors. False explanations for the term involve the gas masks worn by the riot police in that era, or the pigs in charge of George Orwell's Animal Farm.
While police officers usually don't mind being called "cops," they aren't usually fond of the term "pig." A policeman's lot is not an 'appy one.