Along these lines, I never hear “sanguine” or “phlegmatic.” “Gee, you seem quite sanguine today!”
In America only: Solder
In parts of America only: Herbs
Undulate. Slither or wiggle work better in speech.
“Publius”
I’ve known it as the pseudonym under which The Federalist Papers were written. I did not hear it pronounced out loud until it was used in an episode of Scandal.
Is this just a joke about different pronunciations Americans have?
The phrase “tu quoque” I think I’ve never heard aloud, I would have no idea how to correctly pronounce it.
When we studied Dickens in high school our English teacher made sure we said “vittles”, and I have since felt like I’m the only person who knows that.
I sometimes read about a US city called “Decatur.” It looks kind of French so I’m not sure how to say it, but I’ve never bothered to look it up.
I don’t think I’ve ever heard the word ubiquitous used in real life.
Usually duh KAY ter. but teh first syllable is almost more of a d alone
Mr. Spock was known to say “sanguine”. Come to think of it, Mr. Spock probably used a lot of English words that no English-speaking IRL earth human ever said aloud.
ETA: “Sanguine” came up in a college English Composition class that I took. It was in some essay that we read, but there was a spoken discussion of the word in the class (because various students asked what it meant). The teacher challenged us to look it up in a dictionary for homework.
Following day, one person reported that he found two definitions: One seemed off-the wall and unrelated to anything in the essay so he ignored it; the other definition seemed more relevant, but he had already forgotten it.
The “irrelevant” meaning of “sanguine” is
blood-thirsty
Newspaper words seldom seen or heard elsewhere: solons, fracas.
Multifaceted.
Beneficial.
And, as someone pointed out to me: Refreshing. “Oh my, this beverage is refreshing!” No one talks like that.
Huh. It’s use is quite ubiquitous in my day-to-day speech.
There’s always one.
Really? Well I was off by a long shot - in my head it came out as “de kat UR.” Now I won’t be embarrassed if I ever go there.
Tsk tsk, if you’re going to take on us Aussies, at least use the correct form of “its” 
{looks at username}~tsk tsk indeed!! 
I’ll admit to occasionally using sanguine in conversations with its usual meaning of “hopeful”. However, sanguinary means “bloody”, which is confusing if you don’t understand the origin of the words.
A coworker once used chimerical in conversation. It kind of flummoxed me, since I’d never heard anyone speak it before. Also, I’d gotten the wrong idea of its pronunciation based on its spelling.
About 50 years ago, I was babysitting for my cousins. The boy, then about 7, had just learned the word “bosom.” For the entire time I was there, every other word out of his mouth was “bosom.” To this day, I associate that word with him.
Lodonderry Air is the melody now used for Danny Boy. The lyrics quoted are from massive international hit song early in the 20th century that made the melody well known: my grandparents had the sheet music. Since, as a child, I found that sheet music before I heard “Danny Boy”, I’ve had Bosoms stuck in my mind ever since.