talk through the back of (one's) neck
To exaggerate, often to the point of making false or illogical statements. The imagery of the phrase emphasizes that one is not speaking in a straightforward, truthful manner. Oh, you can't trust a word Joe says, he always talks through the back of his neck. Management may be happy with our numbers, but there's no way they give our whole department the day off on Friday—quit talking through the back of your neck.