This piece first ran six years ago. It’s a worthy repeat on a day when I am very tied up in other things. I will explain shortly.
Comprenez-vous? Since language is equal parts communication of ideas and status, conversational bits of French have long been a handy way to say, “I am educated.” French was used as the court language of England from the Norman Conquest in 1066 until Henry V in 1413 (which, as the father of high English, has a lot to do with why Shakespeare gave him a good treatment). An estimated 28% of English words are French in origin, but the words and phrases absorbed directly are the ones that set you apart. They’re still used in the UK, at least in high-toned magazines like the Economist, but in the US it’s more likely to come off as obnoxious.
I have at times called this “Gentlemen’s French”, or what you have to know to read old or educated books. Naturally, fine ladies can use them for the same purpose, so the term seems rather stale. I’m leaning towards “Cross-Channel French” or “French Across the Water” to include Americans, even if we rarely use it as the British upper class still do.
You may prefer to think of these words and phrases as “Cocktail French”, so pour something into stemware and grab a piece of cheese to get into the mood. Here is my list of French words I think every English speaker should know – for the fun, if not the hoity-toit.

Henry V, King of England, Heir and Regent of France and Lord of Ireland (Harry Hotspur to his friends)
à la mode – This usually means pie with a scoop of ice cream in the US, but it really means “trendy” or “in the trend”. Using it that way is very “a la mode,” no?
adieu – Useful because there isn’t an equivalent in English. Literally “to God”, it means you will never be seen again, or at least not for a very long time. It is a dramatic substitute for the daily “au revoir” which is more “see ya later” or “until I see you again”.
Allons-y! – “Let’s go!” If you’re a Doctor Who fan, you should know this.
ancien régime – “Old order,” usually a government or system that doesn’t exist anymore or you want to dismiss as such.
au contraire – “On the contrary.” This phrase has no English use other than to convey status or be silly.
au gratin – Literally “with gratings”, for those times when “with cheese on top” just doesn’t sound as cool.
avant-garde – “Forward guard,” or first to attack. It’s used for the leading edge of hipness or technology. What’s funny is the English word “vanguard” is a corruption.
beau geste – “Beautiful gesture,” which is pretty rare but I like it.
belles-lettres – “Fine letters,” or writing for the artistic value of it (ie, not this).
blasé – Jaded or overly familiar, in English it also conveys boredom. Too cool for the room.
bon vivant – A person of a “happy life”, usually a foodie with a little more je nais se quoi.
canard – A complicated word that literally means “duck”. It somehow became slang for “newspaper” and then “unfounded rumor or story”. It’s evolving to mean “old saying.”
carte blanche – “White card” or a blank check. Unlimited authority.
c’est la vie! – “That’s life!” used to convey resignation to a bad situation. In English it can be more joyous than its original use, which is strange.
cherchez la femme – “Look for the woman.” This phrase describes the course of action when a man is acting out of character or suspiciously, such as when he has an affair to cover up. It can also be used in an incredibly sexist way, and often is. It’s probably best to avoid this one.
coup de grâce – “Blow of mercy,” the final shot that takes out an opponent rather than let them suffer. It’s part of the rich use of the word “coup,” meaning “blow” or “hit” that has taken on a life of its own in English. We use the word “coup” to mean “success,” as in “He pulled off a real coup with that.” It’s really strange.
de rigueur – “Required.” There’s no need for this word, either.
déclassé – lower social class. French makes it more dismissive.
du jour – “Of the day”, a trend or item quickly forgotten.
enfant terrible – “Terrible child,” or “bad boy.” Used often to describe a newcomer who disrupts a scene. It’s usually a compliment, but I can think of one person who might deserve this as an insult.
esprit de corps – “Spirit of the group.” It just sounds good.
fait accompli – “Accomplished fact”, usually used to describe an event that is essentially but not literally finished.
faux pas – “False Step.” Used to mean a social gaffe, such as using these phrases incorrectly.
femme fatale – “Deadly woman.” Any examples I would give would be used against me.
force majeure – “Major force,” which in insurance means an act of God or weather. Conversationally it means “irresistible” or even “charismatic”.
haute couture – “High sewing” or nice threads. It doesn’t sound so great outside of French, does it?
je ne sais quoi – “I don’t know what” or a “certain something”. It’s considered a “faux pas” to say “certain je ne sais quoi” as if “certain” is repeated (as per the less literal translation).
laissez-faire – “Let do,” it’s come to mean “hands off management”.
laissez les bons temps rouler – “Let the good times roll” is also “vollez vous les bon temps rouler” (more literally “You let the good times roll”). It’s really Cajun French from Louisiana, so it’s not Gentleman’s French at all. But we’re all friends here.
léger de main – or legerdemain, it’s supposed to mean “light of hand” but isn’t actually French. “Sleight of hand” or magic trick is how it is used.
milieu – “Middle,” it’s come to mean the social environment in English.
n’est-ce pas? – “Isn’t it true?” Often at the end of a statement. Very useful if you forgot to add Gentlemen’s French in the middle of your sentence and don’t sound as smart as you wanted to at first.
nom de guerre – Literally “War name,” but this has become useful for all kinds of false names and handles. It shows up in gaming sometimes.
nom de plume – If the pen is mightier than the sword, certainly a “pen name” for an anonymous writer is better than a nom de guerre. It literally means “name of the feather,” but we sadly do not use quills any longer.
nouveau riche – “Newly rich,” which is to say people with money and not taste. “Arriviste” or “new arrival” is more dismissive, but not by a lot.
objet d’art – When “tchotchky” just doesn’t have enough class.
parvenu – Upstart. Also dismissive, like many of these words and phrases.
pièce de résistance – “A piece that resists” or main delight of the meal. This is not actually proper French, so it’s more than a little strange.
pied-à-terre – “Foot on the ground”, it means a place to stay. Usually scoring a hotel room as a basecamp to explore a city, and largely unnecessary phrase by any means.
plus ça change, plus c’est la même chose – “The more things change, the more they stay the same.” The first part is used for shorthand, it’s rarely all said out loud.
raconteur – Literally “storyteller,” it’s used to describe a good conversationalist who knows a lot of odd things. I keep hoping someone will call me this.
raison d’être- “Reason for being,” or purpose in life.
rapprochement – Establishment of friendly relations, usually used to describe the end of a battle or snit between two people who really don’t like each other.
répondez s’il vous plait – aka RSVP, it just means “Please respond” (but in a high-class kinda way).
sang-froid – “Cold blood,” it’s used to describe someone cool under pressure.
savoir-faire – “Know how,” or the ability to respond to any situation.
soupçon – “A small amount”, it is used to describe just enough to be noticed but not enough to be useful, such as “a soupçon of relief.”
tête-à-tête – “Head to head,” or intimate conversation. Like many French words which came to us through the language’s standing as the language of diplomacy, this implies a summit of the powerful as it is most commonly used.
tour de force – “Feat of strength,” it’s evolved to mean more like “masterpiece”.
vis-à-vis – “face to face”, it means “with respect to” or “by comparison to”. It’s also not actual French, but may be an old dialect.
vive la différence! – “What a difference!” usually used for gender roles but is becoming a comparison between any two groups. It is often used incorrectly to mean a change in one thing over time, but that is a serious faux pas.
Voulez-vous coucher avec moi? – “Will you sleep with me?” Supposedly, French women will not slap you if you ask them this upon first meeting. I have never been there and I am far too shy to try anyway.
There are many more, I am sure, but this is a list of the ones that I feel are still distinctly non-English words which have not been absorbed fully into everyday speech. They are the ones I want my kids to know just so that they are as obnoxious as their old man. Any additions or corrections you can think of?