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An intensifier used to emphasize surprise, shock, anger, disgust, etc. How on earth am I supposed to read this entire book in one week? What on earth were you thinking, driving across the state in the middle of the night?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
on earthand in creation; in the world
Fig. really; indeed; in fact. (Used as an intensifier after who, what, when, where, how.) What on earth do you mean?' How in creation do you expect me to do that? Who in the world do you think you are?
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
1. Also, in creation; in the world. Ever, anywhere, of all possible things. These phrases are all used for emphasis in questions or, less often, in a negative context. For example, What on earth is he doing with a spade? or Where in creation did that child go? or How in the world do you expect me to carry all those bags? [Late 1700s]
2. like nothing on earth. Incomparable. For example, That perfume smells like nothing on earth, or Her new hair color is like nothing on earth. [c. 1900]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Among all the possibilities: Why on earth did you put on that outfit?
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.