Meaning- To work very hard
Sentence- Nowadays Ram is burning the midnight oil as he wants to score good marks in examinations.

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To burn the midnight oil

Meaning :
It literally means to stay up working hard late at night

Sentence :
If you burn the midnight oil night after night, you'll probably become ill.

More about this :
It was probably derived because in olden times people used to use oil lamp at night. To work you need light and lamp can provide you that light.
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