There are two free dictionaries in Mobipocket format loaded here, under that "E-Books" link above. Here're the direct links:
https://www.mobileread.com/forums/showthread.php?t=16419
https://www.mobileread.com/forums/showthread.php?t=12000
I have loaded the GNU Collaborative International Dictionary of English (GCIDE) into mine (thanks to Adam B. for making it available) while deciding which commercial dictionary to buy. As mentioned in one of the above two threads, it doesn't always seem to manage things like plurals and variations when doing a lookup, or I just don't always find the words I'm looking up. Saying that, it works pretty good for a free dictionary, and better if you don't just assume that because it failed on a word in a text you are currently reading, that it doesn't have the word. As an example, if it doesn't fine "affability", you could try "affable" instead. Try it out. You might find it to be exactly what you're after.
To try it out, you should just have to download it to your PC (assuming 'PC') and open it in the Mobipocket Reader. Assuming you've set the software up with your Iliad as a registered reading device, you should just be able to 'sync' or 'Send' it to your Iliad (this is the way it works with the Iliad V2 and the Mobipocket Reader software, so I am making several assumptions here). If you do not use the Mobipocket Reader, I am
guessing that you just place it in the Mobipocket books folder on the Iliad (or your card if that's where you store your books). (Sorry, this is all a bit vague, and I probably should have left this bit for someone else - you'll either know what I mean, or you'll go "Huh?!" and have to wait for someone with a bit more nous than me to come along
).
Cheers,
Marc