coucou: contrary to what this might sound like to an English speaker, this simply means “hi,” generally affectionately said to or by a good friend or family member. I usually experienced this Skyping or texting. I still feel totally bizarre saying it, as opposed to “salut.” Bonjour I found I mainly used when greeting people I didn’t know at all, or when greeting staff in a business context, like a store or restaurant.
bises or bisous: kisses, mainly experienced by me with friends as an affectionate form of saying goodbye while Skyping or texting. In person, you actually *do* the kissing. On the subject of kisses as a form of greeting, when meeting someone for the first time who is a good friend or family member of a good friend, I’ve found you generally kiss the women hello on both cheeks (on se fait la bise), and at the end of time spent together, assuming it was amicable, you will often kiss both the women and the men good-bye on both cheeks. I’ve also generally seen guys who know each other well, including fairly testosterone types like policemen and construction workers, kiss each other on both cheeks to say hello. It’s kind of hot if I say so myself, but that’s cause I come from physical-contact-phobic America, where that sort of thing just isn’t done.
descendre: generally means going down such as in an elevator, but in a train, it means getting off (je descends du train/I’m getting off the train.)
sur: generally means “on” something (except when you’re talking about a street address, see next), but also used when saying you’re going or returning somewhere, i.e. je rentre sur New York (I’m going back to New York.)
dans: when you want to say an address or place is located ON a particular street, friends tell me you say dans, as in dans la rue St. Denis. I was told once long ago you could say “sur,” but apparently this isn’t as common.
dak: text-ese for d’accord, or ok. You can also simply text “ok,” but dak looks super cool.
c: text-ese for c’est, as in c bon (c’est bon). When you recite the alphabet, “c” sounds like “say” or “c’est.” Less is more!
trankil: text-ese for tranquille. If you text someone how they’re doing and they text back trankil, it doesn’t necessarily mean they’re in a stupor listening to Himalayan singing bowls, it mainly means they’re chilling.
bonne soirée: good evening, when saying good-bye to someone, as opposed to bon soir (saying hello to someone in the evening) or bonne nuit. As an English speaker I can’t hear “soiree” without imagining some debauched all-nighter. For a French speaker, soirée could mean that, but mainly it simply means “evening.” Once on this trip I said “bonne nuit” to someone I didn’t know while leaving an elevator. French friends later told me that while not a gross error, this was a slightly more intimate way of saying good night to someone and more appropriate if you’re, say, crashing at a friend’s home. Which leads to the next clarification...
coucher vs. se coucher: if you say “on va se coucher,” this means we’re or you’re or everyone’s going to bed. If you say, “on va coucher,” (according to some Parisian acquaintances after a few drinks) this means you’re going to bed, with sleep a mere afterthought. Which leads to the last clarification....
préliminaires: preliminaries of course, but in a particular context, foreplay. Though given French subtlety and nuance, possibly also includes sweet nothings, a picked up dinner tab, kissing and other activities leading to the main event. Francophone friends, please correct me if I’m wrong.