Q:

Meaning and origin of "get a grip"

What does get a grip mean and where does it come from? I think it is usually addressed to someone who doesn't understand the obvious, like in the third paragraph of this book review:

I am amazed at the 1-star ratings by "intellectuals" who charge that this book stands on very shaky philosophic ground, and that it does not live up to the high caliber of true scientific studies into communication fundamentals and/or gender differences. Get a grip! That's not the purpose of the book! 

A:

"Get a grip" actually has two meanings.
When a person has let their emotions take control of their actions and thoughts, often times people will tell them to "get a grip" - to get their emotions under control.
From thefreedictionary.com:

Get a grip (on yourself): to make an effort to control your emotions
    and behave more calmly.  (ex. Come on, get a grip, we've got an
    important meeting in five minutes.  I just think he ought to get a
    grip on himself - he's behaving like a child.) 

Get a grip (on yourself) (spoken): to control your emotions.  (ex. I
  know it's hard, but get a grip on yourself and tell me what you saw. 
  Oh, get a grip, Tess! It's really not as bad as you think.)

Additionally, "get a grip" can also refer to gaining an understanding:

Get a grip (on something): to understand how to deal with something. 
  (ex. The program will have helpful tips on how to get a grip on your
  finances. Something is obviously not right in our organization, and we
  must get a grip on the problem.)

As to the origination of the idiom, that I'm not sure of.

A:

It's a shortened version of "Get a grip on reality".  It means that the speaker believes that the person being spoken to is not thinking straight: that they've lost touch with reality.  They are suggesting that you regain contact with reality.
It usually implies that the speaker thinks that your opinion is very poorly considered.

