'Drop the subject', stop talking about whatever you are talking about. Somewhat interchangeable with the other sentences you've written.

Hello, It is the same as Get of the subject or change the subject, when you don't feel like going on talking about something in particular and rather drop it to speak about something else .

well, sometimes people talk about something you wont like to or is to personal or makes you mad, so if your kind you'll say "can we drop this subject" but if you're feeling mad already you'll say "drop it, dude"

It means to end/stop a discussion. For example, "We've been arguing about this for too long, can we drop the subject?"