I was inspired to track this down for the "thee/thou/thy" thread, but it didn't quite fit there. The book is called "The Lost Art of Profanity" by Burges Johnson, 1948, Bobbs Merrill Company; a fine and readable book on the subject, complete with a foreward by H.L. Menken. In the chapter "The Oath Interjectional (or exclaiming)" , well, I'll let the author speak for himself."...the phrase, "What the hell are you up to?" is not seemly for social puropses because it ends with a preposition. A more seemly arrangement would be "To what the hell are you up?" or better sitll "Up to what the hell are you?" But it is hard to make a purist of a man in the midst of ejaculation, though I have known this to happen in a really vicious game of croquet." Enjoy.
In service to variety in language, Mary