for what reason on earth ... for what reason under the sun ... it's not quite right, is it? What are the common expressions with "under the sun"?Hi. "What on Earth" is a very common and set expression, but "What under the sun did he do that for?" makes sense and sounds great to me. Kind of creative, too!
how, why, where, who, etc. on earth
(informal) used to emphasize the question you are asking when you are surprised or angry or cannot think of an obvious answer
- What on earth are you doing?
- How on earth can she afford that?
under the sun
used to emphasize that you are talking about a very large number of things
- We talked about everything under the sun.
- I've been called all the names under the sun!