"ba1 po2" in Mandarin; "baat3 po6" in Cantonese; "BAHT-poh" in more common Cantonese romanization.
It's literally, "the eigth Great Aunt" (think, large family = gossiping women). It means "gossiping bitch" and is considered rude/vulgar/offensive. It's more of a Cantonese term than a Mandarin term. I've heard of it referring to both a male or a female, even though it's a female title. It refers to someone who snitches, spreads rumour, doesn't grasp discreetness, otherwise speaks of things that are completely none of his or her business and/or just a jerk in general.
Sometimes, the modifier '死' (M: "si3"; C: "sei2"; common: "say"; literally "dead", but equivalent to the adverb "fucking") is added in front of it to increase the offensiveness.
Example sentence: 隔壁的八婆最近很喜歡跟大姊聊天。
"Recently, the gossiping bitch next door seems to really like chatting with my eldest sister."