Reluctantly, he decided to drop the physics course. How do you add a parenthical definition to word, drop?

Answer 1

He decided to drop (remove himself from) the physics course.

Answer 2

If you're trying to class up your paper a little bit, add a footnote. It's an easy but impressive way to annotate any text you've written. There's an option in Microsoft Word that will do it for you.

Answer 3

use a synonym in parentheses.
(abandon)(repudiate)