Definition of on earth
1 : in the world <It's the tallest building on earth.> <She said she wouldn't marry him if he were the last man on earth.> <He says that nothing on earth will change his mind.>
2 —used to make a question more forceful <Why on earth did you do that?> <What on earth is he talking about?> <Where on earth are my keys?>
Seen and Heard
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