"C'est n'importe quoi."
Translation:That is nonsense!
Technically, you'd be right. But the phrase « n'importe quoi » has a pretty negative connotation, and it's only ever used to refer to things that are less than desirable. "Nonsense" or "who knows what" are better translations.
For example, take the sentence, « Il fait n'importe quoi. » One might assume it means something like "He does anything," but in reality, the translation is closer to "He's doing who knows what." (such as getting himself into trouble)
It's an other use of it, which means more something like "nonsense", and has for sure a negative connotation. In general, "n'importe quoi" has this negative connotation, that's why we don't use it that much with the meaning you're proposing. It is the same construction as "n'importe qui" or "n'importe quand", but it bot really the sale semantic. In your example, to express that we don't care about what will be serve for breakfast, we would more probably use "je m'en fiche" than "n'importe quoi". Some other examples:
"c'est n'importe quoi" : it's nonsense
"tu fais n'importe quoi" : (I'm not sure how to translate it in an idiomatic ways so i'll explain) no one understands why you're doing it, it doesn't make any sense, it's probably very stupid, dangerous or useless.
"Je mangerais n'importe quoi" : I would eat anything (we would probably understand that you are so hungry that you would literally eat anything we'll give you, even not very edible stuffs...)
That is helpful. Thanks. As someone with a language degree, I have to wonder if this particular phrase varies by region and by age. My ex would be about 67 now, and he is French. He always used it to mean "It doesn't matter at all." - Which would equate a modern "Whatever." As all language does, idiomatic expressions evolve with generations. It could well be that someone in their 40s may use it very differently..
It's in the lesson on idioms, so by now I understand that Duo wants an equivalent English (usually American) idiom and not a word-for-word translation. I looked here: https://www.thoughtco.com/nimporte-quoi-1371318 "That's nonsense" seems a good fit here, or "whatever" but neither are truly idioms in English. Freer and more amusing idiomatic options might be found here: http://mentalfloss.com/article/64162/12-old-timey-ways-saying-nonsense