on earth
idiom.idiom. Used to make a question stronger, especially when you are surprised, confused, or angry.
idiom. An idiomatic intensifier used within wh-questions to express strong emotion, such as surprise, frustration, or disbelief.
What on earth is that noise?
Why on earth did you sell the car without telling me first?
How on earth the ancient engineers managed to construct such a monumental aqueduct with rudimentary tools remains a subject of intense scholarly debate.
This phrase follows a wh-word (what, why, how, etc.) to add emphasis to a question.
Introduce this as a common, polite way to intensify a question, contrasting it with the much more informal and potentially offensive alternative 'the hell'.
On earth what are you doing?What on earth are you doing?The phrase 'on earth' must come directly after the question word (like 'what', 'why', or 'how').