There's a good Google Books dataset for doing exactly that, and you can doing something in Hive to get you the lowest-entropy n-grams (with something like https://github.com/lsb/text-entropy/blob/master/passphrase-s... ), and then you can the low-entropy n-grams into a Bloom filter (like http://www.leebutterman.com/passphrase-safety/how-it-works.h... ) and your enormous corpus gets fitted into a few dozen megs of memory.

(That sounds like a cool idea, hit me up if you're game for hacking on something like that)