Most *nix systems come with a dictionary of words, commonly in /usr/dict/words or /usr/share/dict/words.

The common spelling utility, ispell, also comes with its own dictionaries, although the format isn't quite as simple as that of /usr/dict/words. If you have ispell installed, then you might want to glance at /usr/lib/ispell or /usr/local/lib/ispell to see if you can spot them. (Look for .hash files).

These obviously don't contain word definitions or roots, just the words themselves. However often you can infer the root word using English rules. Again, the ispell source code would probably be a useful start here, since it does exactly that.

Cheers,
Paul