Undreamed-of describes something so big, so good, or so far beyond what people expected that nobody had even imagined it before. 'Undreamed-of wealth' is not just a lot of money — it is more money than the person ever pictured, even in their most hopeful moment. It sits before a noun: 'undreamed-of freedom', 'undreamed-of speeds', 'undreamed-of opportunities'. The picture inside it is the point. A dream is where you keep your biggest hopes, the ones you do not expect to get. So if something is undreamed-of, it has beaten even that — it has gone past the top of your imagination, not just past your expectation. That is why the word almost always attaches to good things, and to big changes: new technology, sudden riches, a life that changed completely. The word is literary and a bit old-fashioned. You will meet it in books, in history writing, in the kind of newspaper article that wants to sound grand. In everyday talk people say 'I never imagined' or 'beyond my wildest dreams' instead. You will also see the spelling 'undreamt-of', mostly in British writing; both are correct and mean exactly the same thing. Though actually, the hyphen wobbles. Some writers put 'undreamed of' with no hyphen, especially after a verb: 'riches undreamed of a century ago'. Before a noun the hyphen is normal and safer.

做夢也想不到的、始料未及的（多用於好事，如「做夢也想不到的財富」）