Are foreign language expressions OK for this exercise? How about stuff like sine non qua, gestalt, and ci-devant?
Dichotomy?
Although, I once used this word in an impromptu presentation at work and only maybe two people in the room knew what the hell I was talking about. We’re talking a room of about 20 office professional types, aged maybe 30 to 40 years old.
My boss told me later that I should gauge my audience and use more appropriate wording! :dubious:
Dichotomy
Yep. If you mention “the Defenestration of Prague”, they can say “which one?” What a town.
I had an English teacher in 9th grade (I think) who would have an in-class spelling bee every couple weeks for each new block of vocabulary words in our textbooks. For anybody left standing at the end, she had a list of French words that would end things pretty quickly. And it was the same words every time. So a few of us survived the first bee, and then nobody knew how to spell “lingerie”. Next time, same thing. Someone finally got “lingerie”, and next was "hors d’oeuvre ". And that one stumped us for the rest of the year.
In her entire career, only one student ever got through the list; my brother, who took her class two years ahead of me.
Not many teenagers, I reckon, have cause to know of “escrow”, but plenty of adults do.
I have a pretty good vocabulary, and I’ve never even seen “ci-devant.” And isn’t it “sine qua non”?
A few more I remember learning as an adult:
abject
opprobrium, opprobrious, and approbation (easy to mix up the senses of these words)
sapient
salient
strident
Not sure why I’m coming up with so many adjectives 
Here’s the definition:
Yes it is. See how tricky 
“Defenestrate” has become the Millard Fillmore of English words. That is, it’s famous for being obscure and, thus, no longer obscure. I’d guess more than half my friends are familiar with the word.
I was in the Czech Republic over Easter, and told my (12 year old) son I had been in Prague. He asked me if I shouldn’t have tossed anyone out the window while I was there. History is a great way to learn words, and that’s one of those that you don’t tend to forget once you know it.
Impetuous and ostentatious are words that nobody uses in everyday conversations? Really? They’re not unusual words at all.
Mind you, it does depend on your audience. For a couple of years I was mostly mixing with people who left school at 16 and never read for pleasure or did anything else that you’d consider educational, official or not. They were flummoxed by the word ‘fickle.’ They would have been flummoxed by the word flummoxed too, I expect.
How about:
Intuit
Intuitive
Stanza
Hirsute
Masticate (probably one a lot of people learn in their teens :D)
I actually came across the word “morganatic” in use during the period when the marriage of Prince Charles and Camilla was in the news a few years back.
Assuage. And bonus points if you pronounce it correctly.
I can’t think of a time when I’ve ever said something was impetuous or ostentatious. I know what those words mean, mind you, but I’ve never said nor heard someone use it in a way that didn’t stand out. They may not be unusual, but they’re rare.
Take “masticate” for example. I know what that means and I suspect a lot of adult do as well. But I’d never use it. I’d just say “chew” instead. In fact, the only times I’ve said “masticate” or heard it were in masturbation jokes, e.g. “He masticated the whole time we were at dinner!”
I love “escrow” because that makes perfect sense as for why a pre-adult wouldn’t know it. Adults say that word fairly often but a kid couldn’t be expected to pick up on it. Intuit and Intuitive are good words for this list, assuming you’re right that pre-adults don’t know them. Those make me go “Wow, people say that all the time. I can’t believe high schoolers wouldn’t have heard that by now.”, which is what I’m after here.
I agree that “masticate” is an odd one to work into conversation, but I hear “ostentatious” commonly enough that it doesn’t stick out to me. “Impetuous” is on the cusp. Definitely heard it worked into conversation, but not as commonly as ostentatious.
I used egregious in a post just today. 
How about heinous? Bonus points for that one being pronounced correctly, too.
ETA: My guide for these words would be if I hesitate slightly before using it around people other than my husband, but do it anyway because dammit, people should read a book once in a while!
My wife and I (32 & 40) were really suprised that a lot of people in their 20s and younger don’t know what RSVP means.
Not the actual french Répondez s’il vous plaît, but the basic Please Respond.
We’ve sent out invites to our son’s birthday party 3 years in a row with rsvp on them to the kids parents and we’re lucky to get a 20% response rate. Some just no-show and some just show up.
We thought they were just being lazy until a couple of them admitted that they didn’t know what it meant. Then a whole bunch of them admitted the same thing.
high five
Wow, I never took the SATS (or went to high school, ftm) but I know what ALL these words mean, have since I was a teen in most cases, and use them from time to time as called for.
Admitedly, I use them more often in writing than in speech, but still. I would have no qualms at all using egregious or exacerbate or intuitive or most of the others in normal conversation. (although, I admit to sometimes dumbing it down depending on the audience, often after I have used some such word and been met with universal incomprehension. :rolleyes:)
I still have a 20 yr old t-shirt with a reproduction of a mural in Austin, Tx. which includes a banner reading, “Austintatious”. Then as now, a solid majority don’t get it on any level. 
And defenestrate has been one of my (and my son’s) favorite words since we discovered it years ago. That is one I will admit to not knowing at a young age. In my defense, how common is the situation calling for its usage? I have yet to manage to work it into conversation. I may get angry with my cat and threaten to throw her off the balcony (never do, of course) but somehow saying “Cat, I will defenestrate you if you don’t shut up and/or stop clawing the furniture!” just doesn’t have the same ring to it.
I thought of a couple more:
Ecclectic
Minutiae
Detritus
Impoverished (this was surprising - I even had a college student once ask me when I used it in conversation, “Ummm… Do you mean poor?”)
*Eclectic.
(Sorry…)
How about inchoate? That’s the last word I remember learning. My next one will be callipygian - what in the world does it mean? I could look it up, but that wouldn’t be as much fun.