A name is a name is name, except when it is a thing. Very few first names ever become common nouns but the process does occur occasionally. In English, the major beneficiaries are John and Mary.
John is one of the most common first names in English, which is reflected in its appearance in standard dictionaries. For example, a john is a toilet, possibly due to Sir John Harringon, who wrote erotic risky poetry and improved the flush toilet of his time. It is also a customer of a prostitute, who is rarely ever mentioned by name. For the same reason, a John Doe is a nameless person while a Johnny Reb is a nameless soldier of the American Confederacy. John Bull is the symbol of Britain created by, surprise, John Arbuthnot, a pamphleteer of the 18th century. It is always unpleasant to receive a Dear John letter, informing you in a rather distant way that your girlfriend has decided to leave you, a World War II phenomenon that proves that distance does not always make a heart grow founder. Finally, the new kid on the block is a Johnny come lately, probably based on old British military term.
Among girl’s names, Mary is the prominent representative. At a bar, you can drink a Bloody Mary or even a Virgin Mary, if you are the designated driver, that is a mixture of vodka and tomato juice and its non-alcoholic healthy cousin, respectively. Every baker uses a bain Marie, which is a double boiler, to melt chocolate, a direct transfer from the French term, or can make a Mary Jane cake, a type of pound cake apparently dated from the 1950’s. Continuing that thread, being a plain Jane, i.e. having an unremarkable physical appearance, origin unknown, is a bit of a mixed blessing.
A few other names have also entered general language. Everybody knows that Uncle Sam is not a relative but instead a synonym for the U.S government. Likewise, a lazy Susan does not lack character but instead is useful for serving food on a large table. Once again, the origin is unclear. In these cases, every Tom, Dick and Harry will understand what the words mean, which is the point of the matter.