This is just for fun. :)
I think I can say I have a pretty extensive vocabulary. I read a dictionary cover to cover when I was a kid (not that I've retained it all ;) ).
So, I didn't expect the dictionary to know much that I didn't know. :) Occasionally, though, I do run into something. It's often something obsolete, really specialized, or of foreign derivation.
Here's one from today:
Verst: (noun) a Russian unit of measure, roughly two-thirds of a mile (slightly over a kilometer)
How about you?