The prefix for- was still in Chaucer's time "productive" (that is, it was still kosher to stick it in front of a verb and make a new verb on the fly, as you can in modern English with un- or re-.) If a verb meant "to do X," then with the for- prefix it meant "to do X to the point of destruction." So when the Miller is "fordronken" he's not just drunk, he's drunk to the point of destruction. Hopelessly drunk.
The prefix is still around, in fossil form -- mostly in past participles, such as fordone, forspent, forlorn. But you can't make up new words with it any more. You can't say your email account has been forspammed, or that your eggs this morning were forfried. Which is a shame, I think.
In the Book of the Duchess, Chaucer describes a grieving widow as "forweped and forwaked," and I can't think how you could say the same thing with anything like such economy and vividness in Modern English. In fact, while "all cried out" might work for "forweped," I have a hard time thinking of how to express "forwaked" at all. "Exhausted for lack of sleep?"