“Drop it down” or “jot it down”?

The correct phrase is “jot it down”.

“To jot down” is an English phrasal verb which means “to write a note”.

A common mispronunciation (particularly by Hong Kong Chinese) is “drop down” or “drop it down“.

Some correct examples:

This point is very important. I think you should jot it down.

Before you call her, you should jot down some notes about what you want to say.