Drop me a line

短簡,短函[S]
casual reply

Drop me a line when you arrive.

你到達以後給我來一封信。

"drop me a line" means "write me a sentance" (meaning, respond with a short reply)

it usually come with "when you have a time".
totally, we put in the end of our message, "drop me a line when you have time".

Drop me a line=投下我線

Drop me a line 點解?

即簡單地給我一個回覆.
例:
Please drop me a line regarding my request.

請寫信(不用太長)給我吧。