Learn English – “A tax is a tax is a tax.” What does this sentence mean

T.E.D. has a fine answer, just wanted to say it my way. "Proverbial" means "having to do with a proverb", or as T.E.D. says "as heard in a proverb". There are many well-known proverbs in colloquial English. For example, there is one, "people in glass houses shouldn't throw stones", meaning that people in a precarious moral position shouldn't point out the failings of others. English proverbs are generally allegorical, similar to a biblical parable (and some parables, such as the parable of the mustard seed, and other biblical stories like David and Goliath, have remained popular in English even as the culture trends toward the secular). Some are poetic, such as "beer before liquor, never been sicker" (meaning if you start out an evening drinking beer, then progress to hard liquor, you'll have a bad hangover in the morning), and contrary to the connotation of the word "proverb" as imparting ancient wisdom, proverbs are invented all the time ("loose lips sink ships" was coined in World War II, in the months leading up to the D-Day invasion; it was such a big operation with so much of the military and industry involved that the U.S. Government basically asked the entire country to keep whatever they knew or suspected about it secret).

So, a person who talks about "the proverbial man in a glass house" is referring to the proverb in my first example. Where the proverb is known, its imagery can be evoked without recounting it fully.

"Proverbial" can also be used as a synonym for "idiomatic", having a similar meaning but relating instead to idioms. Idioms are popular, somewhat metaphoric sayings regarding common situations; for example:

• Up a creek without a paddle (in trouble with no way of helping yourself)
• Out on a limb (at serious risk for injury to body, reputation, or pride)
• Getting up on the wrong side of the bed (starting the day in a bad mood)
• Getting cold feet (becoming nervous about something one is about to do)
• The pot calling the kettle black (calling someone else out for a flaw you obviously possess yourself)

These sayings can also be referred to "proverbially", even though they're idioms; you may hear of being "up the proverbial creek", which is simply an alternate way of saying "up a creek without a paddle" as the original term has become slightly cliche.

This is a somewhat archaic construction. The more modern way to say the same thing would be

It took the prince three days to die.

In my mind, the "was [timespan] dying" construct does serve to emphasize the fact that the person was dying the whole time, rather than "working up to it" in a sense.

(And just to be perfectly clear, yes, it does mean that the prince is now dead.)