why on Earth

Interjection

idiomatic

(idiomatic) used to add emphasis to "why" when beginning a question.

Japanese Meaning

一体全体なぜ / いったいどうして / どうして一体全体

Why on Earth would you bring a surfboard to a mountain cabin?

いったいなぜ山の小屋にサーフボードを持ってくるのですか？

Sense（1）

(idiomatic) used to add emphasis to "why" when beginning a question.

Why on Earth are you pouring vinegar in your drink?