"Non esiste" or "Non esiste proprio" (Lit.: it doesn't exist / it doesn't exist, really)

Let's pretend that you don't want to go out with the friend who asked you for a spritz together. If you're close enough, this sentence allows you to refuse or oppose in a informal and quite straight/rude way.

"Allora, ci andiamo a prendere 'sto spritz?" "Non esiste proprio, sono troppo occupato"

"So, let's grab a spritz?" "Absolutely not, I'm too busy"

Don't use it in formal circumstances, for example with your boss. If he/she asks you to have a lunchtime together and you can't or whatever, just refuse in a polite way.

"Ma mi faccia il piacere!" or "ma fammi il piacere!"

Well, it literally means "do me a favor..." but actually is a strong and typical way to oppose and argue. There is a huge difference, indeed, between "Potrebbe farmi un piacere, per favore?" (Could you do me a favor, please?) and "Ma mi faccia il piacere!" (Oh, shut up! Don't be silly! The bullsh*it you say!). That "ma" leads the musicality of the sentence in a definitely not friendly mood.

"Quanto costa quell'accendino?" "500 euro" "Cosa? Ma mi faccia il piacere! Se lo tenga!"

"How much for that lighter?" "500 euros" "What? Don't be silly! You can keep it with you"

"Ci sta" or "ci sta tutto"

That's fine, it fits well, good idea, I agree. This makes sense and can work, this would do the trick. Especially when one answers a proposal.

Ex.:

"Sono le sei, andiamo a farci uno spritz" "Sì, ci sta"

"It's six o' clock, let's grab some spritz" "Yes, Ok. It sounds good, it seems appropriate and a good idea as well and I definitely agree. Let's go"

"Mollami" or "dammi tregua"

Leave me alone, I want some peace, I don't want to know anything about this, give me some break. When someone is pushing or annoying.

Ex.:

"Hai deciso se vuoi uscire con me, stasera?" "E mollami un attimo, per favore. Sono occupato adesso..."

"Did you decide if you want to go out with me, tonight?" "Omg, leave me alone, please. I'm busy, now"

"Ma che ti dice il cervello?" Lit.: "What (the hell) your brain is telling you?" Meaning: "Are you mad, dumb or what? How can you think what you did is good, polite, appropriate, safe etc...?"

"Fico", "ficata" or "figata", "Grande"...all these idioms mean "cool!" "great!" etc...

And then, a lot of local idioms, that became popular in the whole country and often require you to imitate the right accent. In Rome, for example:

"Maché, davéro?" (Ma che, davvero?) > "Really? You serious? You kidding me? This can't be true"

"E 'sticazzi?" (Lit.: and those c*cks?) > who the hell cares?