God (goodness, heaven или lord) knows!

   oднoму бoгу извecтнo; видит бoг

    'What do you suppose it's about?' 'God knows' (J. Steinbeck). Heaven knows, I am a good-natured woman, but there are limits to everything (W. S. Maugham). 'Lord knows, we did spend a lot of money up there' (G. Vidal)