Can someone explain the meaning and common usages of this phrase? I hear it often and am not entirely sure I always understand it. Does it mean something like English's “whatever” or “bullshit”? Is it said flippantly? Angrily? I would love to see some examples of situations or usages of the phrase so I can get a better grasp on it.
N'importe quoi can mean Bullshit.
Could be translated in (without sounding as rude as in English in my opinion)
But you can also use it for 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
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.
which could be translated as
The "basic" translation is whatever.
For the more idiomatic use I guess almost literal translations like:
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
even if it is a bit too strong, since "N'importe quoi" can be used also in an ironic/joking context.