A phrase typically, but not always, used in conversation; can also be used in response to a fact, or its opposite, in media; the appropriate response to any sentence including any variation of the descriptor 'bullshit', when the word 'shit' is being used as a verb, and the preceding perceived assertion or premise reflects a known truth or fact. Also used as its reverse as a prank, to generate belief in something patently false and normally unbelievable.
You: Idaho is a state in the USA? You're shitting me!
Them: I shit you not! It's a state!
You: I thought it was an invention of the corn industry.