J. K. Rowling says this about early morning commuters: "Muggles bustled past wearing the hangdog expressions of early morning". While I do understand the meaning of the idiom (the same as 'morgensur' in Danish, meaning 'sorry about having to endure this morning'), I wonder where it really comes from.
The only explanation I've come up with is lifting the dog by the scruff. I have a dog, he looks so pathetic when I do this, I want to hug him immediately (and that's supposed to be a punishment, mind you). I couldn't come up with a better explanation and have failed to find something conclusive on the Web to support my idea.
So, when does a dog hang otherwise? :)