I have a query about the meaning of 'on earth'. Here is an example.
What on earth are you doing under the table?
At here, what is the purpose of 'on earth'?
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What on earth
This expression is used to add emphasis to questions. In other words, you use the expression when you are extremely surprised, angry, or confused about something. The phrase is used in informal English.
You can use "what the hell" instead, which is also informal, but it's impolite.