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 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 somebody a ˈline/ˈnote
(informal) write a short letter, message, etc. to somebody: I dropped her a line inviting her to my birthday party.