Ameliorate and assuage are my faves but I must admit I get to use them a fair amount. Also Krankenschwester (which is german for nurse) which I never get to use now that I no longer live in Germany.
Tomfoolery
Now this is a word that I somehow always manage to use incorrectly (and I suspect I am not alone in that). I don’t know why, but I can never remember that penultimate doesn’t mean “the best” “prime example of” or “ultimate”. Stupid brain.
Yet another vote for defenestrate.
So you don’t get to use it much because your windows don’t open, or it would be *especially *fun to use because your windows don’t open?
I like egregious too, but of the words that contain my name, I prefer gregarious. My favorite underused word is mondegreen.
Anathema. I love that word.
Terpsichorean.
Also, imprimatur.
I’d say “copacetic,” but I do get to use that one fairly often.
antithesis
Just like it.
If you open the window before tossing someone through it, you’re doing it wrong.
Banana hammock. It can either mean men’s tighty whities or men’s Speedos. Okay, it’s two words, but fun as hell to say. 
I thought those were called “meathooks”?
:dubious:
Yes! Just because you can’t help but get a mental picture.
I love to use the word blasphemous. Although I probably don’t use it in exactly the correct way.
Perpendicularity.
I use square , or true, or at a right angle most of the time, but sometimes in a sing-song voice I say “perpendicularity”, just to sound like a smartass.
I forgot “specious.” I rarely get to use that one and it’s one of my faves.
I really don’t know why we can’t have a thread about unusual words without people castigating others as simpletons for not knowing words, or as snobs for knowing and using unusual words. There’s really no good reason for snark in a thread like this.
Callipygian, alas and alack.
Vicissitudes of life is a concept in tort law.
Picayune, is my favorite word. I could use it a lot, but almost nobody understands it.
My favorite word I rarely get to use is a simple one. ‘ta-da’
Unless you are a magician you never get to use it. I one day want to present my design ideas to a client with a flourish and say ‘ta-da!’ That would be awesome I think.
Piss-elegant: a delightfully snarky, pretentious word to mean “pretentious.”