Bullshit

N’importe quoi can mean Bullshit.

— Les poules peuvent voler.
— N’importe quoi !

Could be translated in (without sounding as rude as in English in my opinion)

— Chicken can fly.
— Bullshit!

But you can also use it for all wrong

— Tu as fait n’importe quoi!

Meaning

— You did it all wrong!

(Literally “You did whatever randonmly occurred to you”)

Whatever / I don’t mind

If you want to use it as whatever or I don’t mind, you shall use peu importe or n’importe

— Que veux-tu manger ?
— Peu importe.

— Tu veux t’asseoir devant ou derrière ?
— N’importe.

Meaning

— What do you want to eat?
— Whatever.

— Do you want to sit at the rear or at the front?
— I don’t mind.

What a nonsense

As suggested by RomainVALERI in the comments, n’importe quoi can also be used for pointing out how nonsensical / shameful something is.

— Ils ont réussi à faire de la pub dans les écoles ! C’est n’importe quoi…

which could be translated as

— They managed to put advertising material in schools! What a nonsense…

The “basic” translation is whatever.

For the more idiomatic use I guess almost literal translations like:

• Nobody is thinking about what it is like
• (almost) anything possible
• Whatever (you can imagine)

can give a hint. Moreover the expression is ueds in a negative way, meaning that someone is making almost all he/she can to achieve his/her worst

Some other translation could be

• All the worst you can think of

even if it is a bit too strong, since “N’importe quoi” can be used also in an ironic/joking context.

The actual meaning is meant for someone who doesn’t know what he or she is talking about. ‘N’importe quoi’ literally means ‘Anything’ or ‘Whatever.’ You would normally use it to indicate to a person that they sound ignorant or like an idiot.

Example: When someone gives a ridiculous explanation for something, you would answer: N’importe quoi.

Another example would be if you had no preference for something, then you would answer, N’importe quoi, as in ‘whatever’ or ‘anything.’

I hope this helps.

“N’importe” basically expresses a “non-choice”.
Il ferait n’importe quoi pour elle : He’d do any/everything for her.
Donne lui n’importe quoi et il est heureux : Anything you give him makes him happy.

Note that “n’importe” can be used with other words, with the same “non-choice” meaning :

• N’importe où

  Il irait n’importe où avec elle. He’d go anywhere with her.

• N’importe comment

  Elle s’habille n’importe comment. She can’t dress.

• N’importe quand

  Venez n’importe quand, on est toujours là. Come by whenever you feel like, we are always home.

• N’importe qui

  À qui veux-tu parler? — N’importe qui. Who do you want to talk to? To anyone.
  On ne va pas engager n’importe qui. We can’t hire just anyone.

  Idiomatic expression :
  Ce n’est pas n’importe qui. She/He is an important person.

• N’importe lequel/laquelle/lesquels/lesquelles

  Quelle voiture veux-tu? — N’importe laquelle. Which car do you want? — Any car will do.

• N’importe quel/quelle/quels/quelles

  Ça ne se vend pas dans n’importe quel magasin. They dont sell that just in any shop.