how, what, why, etc. on earth...
how, what, why, etc. on ˈearth...,
how, why, etc. in the ˈworld...(informal) used with questions to express the speaker’s surprise, anger, etc: How on earth did you know I was coming today when I didn’t know myself until the last minute? ♢ Why on earth would anyone give up such a good job?
See also: on