Correspond, write a short letter or note. For example, I hope you'll drop me a line soon. This idiom uses line in the sense of "a few words in writing," a usage first recorded in 1647.

What do drop me a line mean?

: to send (someone) a brief note or message Drop me a line while you're away.

Does drop a line mean call?

drop (one) a line

To contact someone, usually with a letter, note, or phone call.