Why do we say 'How the devil are you'?

[TheFullJenny]

Why do we say 'How the devil are you', why does the devil have anything to do with it?

[jno]

questions used to be asked along the lines of 'how in the devil's name [or God's name, for that matter] did he do that?' Because, I suppose, the English swear all the time - they have long been famous for it; medieval Frenchmen used to call them 'les goddams'. Anyway, your question is a slight variation on that: 'How are you?' with added swearing.

[Quizmonster]

The earliest recorded use of the phrase "How the devil..." was from Caxton - he of printing-press fame - and published in the 1480s, so it's been around for a long time! The 'devil' idea was also attached to when, where, who, what etc questions to add a note of surprise, irritation, dismay and so on.
"How the devil are you?" indicates that the speaker is pleasantly surprised to see you.

[VS Prasad]

In "How the devil are you", the devil is just added for emphasis, in this case reflecting, presumably, that fact that the speaker hasn't seen the spoken to for quite a while, or that something has happened to the spoken to which makes the question a real one, rather than a perfunctory or oratorical one. The speaker could also have said, "How the heck (How the hell) are you?" or any number of similar variations.

http://www.phrases.org.uk/bulletin_board/42/me ssages/920.html