What is the origin of the idiom "plum in one's mouth" and when to use it?

The idiom of the phrase "plum in one's mouth" is uncertain, but it is believed to have emerged in the late 19th or early 20th century in Britain. It is often used in conversations, literature, or discussions about social class, etiquette, or linguistic style. This phrase is mostly used in a lighthearted or humorous context, suggesting admiration or amusement rather than disapproval.