drop somebody a line/note
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 (one) a note
To send one a letter or message. I always try to drop Janet a note around her birthday so that she knows I'm thinking of her.
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.