What does "burn the midnight oil" mean — and why is it funny?

informal

Meaning

To work or study late into the night.

Where it comes from

From the centuries before electricity, when working after dark literally meant burning oil in a lamp.

Why it is funny

The gentle charm is how dated it has become. We still say it while staring at a glowing laptop, solemnly describing our late night by the light of an oil lamp nobody owns any more.

Used in a sentence

"I burned the midnight oil all week to finish the report."