"talking through the back of her neck"

More Wodehouse.
This is from The Code of the Wooster.

What does it mean?

-)

There is a phrase "talking out of your @$$" which is used when a person is speaking about a topic that they know very little about and trying to fool people into thinking that they do. This could be a variation. Maybe this is a British variation? Could you supply some context?

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May be. I am not sure.
This book was written in the 1930's.
Here is the context:

(the aunt says:)

".all I wanted was to tell you to go to an antique shop in the Brompton Road and sneer at a cow creamer".

I did not get her drift. The impression I received was that of an aunt talking through the back of her neck.

(cow creamer is a milk jug).
The reason the aunt sends him to the shop is to lower the price.

I don't think this is a commonly used phrase per se. I think it just means he had no idea what she was on about - it was as bizarre as her voice coming from the back of her neck would have been. I don't think it's the same as the American expression - "Talking out one's ass" or the British experssion "Talking out one's arse", either.

Yes.
Googling the phrase I see the it being used to describe someone who has no idea what he or she is talking about as in:

"How, you may be asking, do I dare to dissent from the opinions of the judge? I dissent because I have read the evidence presented to Hutton, and I put it to you that the judge is noble, learned and talking through the back of his neck."

The more common version in Britain is, I believe, 'talking out of the back of your head'. It means the same.