drop (one) a line
To contact someone, usually with a letter, note, or phone call. I know you'll be busy enjoying yourself, but please, drop me a line on your trip. I'll drop you a line once I get settled in at school.
drop someone a line and drop someone a few lines; drop someone a note
to write a letter or a note to someone. (The line refers to lines of writing.) I dropped Aunt Jane a line last Thanks giving. She usually drops me a few lines around the first of the year. Drop me a note when you get a chance.
drop a line
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.
drop someone a line
send someone a note or letter in a casual manner.drop somebody a ˈline/ˈnote
(informal) write a short letter, message, etc. to somebody: I dropped her a line inviting her to my birthday party.