on earth
Among all the possibilities

[TahlilGaran] Dictionary of Contemporary English ▲

on earth
in any conditions.
What on earth makes you say that?

Why on earth would she ask you to join them?

How on earth did you survive the heat?

Who on earth would want to collect rocks?

Where on earth could Casey have learned such behavior?

Usage notes: used to express great surprise that something could happen or exist

[TahlilGaran] English Idioms Dictionary ▲

on earth
{adv. phr.}, {informal} Of all possible things; ever. - Usually used for emphasis after words that ask questions, as "who", "why", "what", etc.
Where in the world did you find that necktie?

The boys wondered how on earth the mouse got out of the cage.

Betty could not understand what on earth the teacher meant.

See: IN THE WORLD.

[TahlilGaran] English Idioms Dictionary ▲