People call police officers "pigs" to be offensive. The term is used in reference to police officers in attempt to belittle them or insult them. The term has been used in this manner and for these purposes since 1811 when the it was published in the "Dictionary of Buckish Slang."
"Bacon" is another name that is used to offend police officers. Because bacon comes from a pig, the two terms are related. In attempt to be insulting, someone might say "I smell bacon" when police officers are nearby. This particular phrase is also used to warn others that the police are in the area.