Here are a few phrases and words that can make you feel awkward if you don’t know their meanings.
Ç’est parti ! – Here we go / we’re off !
Ça vaut le coup – It’s worth it
On voit que dalle – You can’t see anything
Ça va être chaud ! – It’s going to be tough!
Fais voir – Show me / Let me see
débarquer – to arrive without notice
bosser – to work
bouffer – to eat
bourrer – to stuff, cram
être à la bourre – to be in a hurry
être à mourir d’ennui – to be very boring
faire gaffe – be careful, pay attention
filer à l’anglaise – to leave without saying goodbye / to take a French leave
flipper – to go crazy, flip out
louper – to miss
piquer – to steal
If you can’t remember the name of a thing, you can use ce truc, ce machin, ce bidule or cet engin similarly to the English thingy, thingamajig, whatchamacallit, etc.
What’s-his-name is Monsieur Untel and what’s-her-name is Madame Unetelle. You can also use Machin but it has a more derogative meaning, as if you are saying the person is merely an object.