A Google search shows me that this is from "The Women's Room," by Marilyn French. I haven't read it myself. It was quite a famous book in 1977 and it is very definitely a feminist book. There is no doubt in my mind that it stands for "male chauvinist pig."
The original meaning is now all but forgotten, but before 1970s feminism, "chauvinism" meant excessive, fanatical PATRIOTISM and a belief that everything about one's own COUNTRY is better than other countries. The feminists drew an apt analogy between this and male arrogance about their gender and began speaking of "male chauvinism."
"Pig" is a longstanding derogatory term for men behaving in an arrogant, sexist way. In Somerset Maugham's famous story "Rain" closes with Sadie Thompson shouting "You men! You filthy, dirty pigs! You're all the same, all of you. Pigs! Pigs!"