After using the word "rejoinder" in a comment on Facebook, I thought to myself "that can't be a very frequently used word". I found a website that allows you to see frequency of use of words from the Google Books project.
It allows you to graph the frequency of words or phrases. By way of comparison, "the" is used about 5% of the time (roughly speaking long term average). The word "rejoinder" peaked in usage (at least in books) in the last half of the 19th century, never going above 0.0001748422%. So you can expect to see "the" about once out of every twenty words you read, but "rejoinder" won't likely be seen more often than once out of every half a million words or so.
I don't consider myself to have an exceptionally large vocabulary. Maybe larger than average, but certainly nowhere near some people I know. I was a little surprised by the numbers, though it makes sense. If there were twenty words that were all used 5% of the time, there wouldn't be room for any other words. Still, that 0.0001748422% number was surprisingly low to me.
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