One of the great joys of French is that the amount of words shared between it and English is simply staggering. Even if you have never learned a single word of French, if you look through a French technical or scholarly text (although not a literary or popular one) you can pick out the meaning much of the words and even figure out some of the meaning, since there are so many that are the same. Not necessarily pronounced the same mind you...
But this also speaks to one of the odd quirks of French and English, that many of these words from French exist in English, but on a notably more formal and elevated plane than they do in their native language. It makes translator's job hard, since it is easy to translate word by word, but then you end up with a cold, formal, and excessively literary text, that lacks the charm and simplicity of the original. But what if you do it purposefully? There are plenty of people who love to speckle their writing or quotations with a little je ne sais quoi but that's amateur talk! Why not go all out and throw in the obscure, literary, little-used, literary words that exist in English but are almost never used, but pop up somewhat reasonably frequently in a French text! Great for alternatively either annoying someone, or appearing sophisticated, or both at once.
Of course, as one might note, a lot of these words don't actually stem directly from French: many of them are from Latin for example. But I've hardly ever seen them in English, if at all, even if they are valid words - but they do make their way through French books!
# 10 - Panaegyric
Definition: something (such as a speech or a piece of writing) that praises someone or something
Perhaps your friends and family won't be eager to give panegyric compliments to you for using the word, but it's a great one nevertheless. It beats out praiseful, boastful, complimentary, laudatory, etc. by a country mile!
9 - Lapidary
Definition: Short, concise, elegant, terse
Most of the time if you look up lapidary you will find the first descriptions to be about stone cutting, some sort of practice of shaping, cutting, and polishing stone. But it also has a meaning of being short, concise, elegant, pithy - seemingly coming from putting inscriptions and writing on stone. While few of the words here are lapidary, and maybe you're betraying its spirit in using them, what a nice upgrade to one's language!
# 8 - Desuetude
Definition: A state of disuse, no longer practiced, no longer used
Most of the words here have fallen into desuetude, but if they're still in the dictionary, they're fair game in my eyes!
# 7 - Sibylline
Definition: Enigmatic, obscure, relating to an oracle or prophet*
Everyone will love this one, for it perfectly matches what you want: something real and true, supposedly at least, but whose meaning and definition is hard to figure out.
# 6 - Thaumaturge
Definition: A miracle-maker, wonder-worker, magician, particularly common in magic or religion
Normally in English you might call a thaumaturge a miracle-maker, with the general credit that English has that the compound of the words generally means you can figure out the concept from the two combined, something which French isn't always as good at (see, say, basement window vs. soupirail). But there's no need to let that stop you, since thaumaturge is a perfectly legitimate English word, and in fact one which even does have a decent specific purpose, religious affairs with wonder-making. But why not use it beyond that, and be a thaumaturge yourself with the wonders and joys you can bring to your social circle.
# 5 - Socle
Definition: Base, foundation, of a colum, pedestal, or statue
Socle gets somewhat more usage in French than in English since in French books it will sometimes be used as an elegant way to refer to the foundation of society, which resposes on a socle. Why not do the same thing in English, and replace foundation - even if, sadly, socle is shorter.
# 4 - Ameliorate
Definition: To improve, to make better
Ameliorate doesn't seem like that fancy of a word to me, but that might be because it is a quite common French one - the equivalent of improve. But so far when I have used it in English at times people tend to not know what it is referring to, and even if they do, it's still a great improvement, particularly since it has 4 syllables instead of 2.
# 3 - Limpid
Definition: Water which is clear, translucent, crystal clear
Limpid doesn't have quite the same ring in English as in French, because it sounds like our word limps, with all of its connotations of staggering along (boiter), or soft and not rigid (mou or flasque). So referring to the limpid water makes you tend to think that the water is stagnant or injured, instead of clear and crystal - but that second meaning exists too! Just don't try to win points for the beauty of the poetry created.
# 2 - Velleity
Definition: A slight hope or inclination, an indefinite and to-be-unrealized project
Even French people don't seem to have heard of their version of velleity, velléité, since I mentioned it was a word which I had trouble remembering and spelling, and the person I mentioned it to hadn't heard of it. But it's around in both English and French, a faint, vague, probably unrealizable hope, a fantasy, a vague sketch, a dream that you have but won't carry out: everybody will only have the slightest velleity of figuring out what you're saying!
Pas besoin d'être Jéremie...
# 1 - Jeremiad
Definition: Lamentation, plaint
And finally a word which will be useful for everyone else around you - jeremiad! Because their jeremiads will be clear to hear, with the long tales of woe about your antics.