Though first referenced by the Oxford English Dictionary in 1811, the slang term "pig" in reference to policemen didn't become popular until the late 1960s. The term was used by protesters at the 1968 Democratic National Convention and subsequently popularized in the media.
A group calling themselves the "Yippies" protested outside the Chicago convention in opposition to the Vietnam War. These protesters carried a small pig as their presidential candidate, and they began calling the police themselves "pigs" when the officers attempted to disband the demonstration. The media headlines declared, "Police Called Pigs" and, according to the School for Champions, the term caught on, especially among younger people and the disenchanted.