English has more words than any other language - around 2 million at the last count - but the average person's vocabulary only spans from around 12,000 to 17,000 words. That leaves a lot of perfectly acceptable words that seem strange to most native speakers, since they're either words with a more common synonym, rarely-used variants of otherwise known prefixes and suffixes, or just simply words that have become obsolete.
Here are a few examples of words that probably aren't part of the average person's vocabulary. It's probably better not to think of this as an exercise in expanding your vocabulary, since the vast majority of people will still have no idea what you're talking about...
Nudiustertian - a word that is often part of other languages, but not in common English, this means "the day before yesterday"
Zenzizenzizenzic - an obsolete form of mathematical notation, from back when numbers would be written out as words
Flibbertigibbet - sounds like the catchphrase of some kid's cartoon character, but in fact it's a legitimate word, meaning "a silly woman"
Syzygy - No vowels, a Zs, 3 Ys a G and an S? That's not a word. Except it is - it's an astrological term for when 3 or more planets are in alignment. How to pronounce it is another story...
Inaniloquenteloquent, this word is about speaking. In fact, it's a combination of the words inane and the -loquent stem, and means "pertaining to idle talk or chatter"
Nebbish - a weak-willed person