drop (one) a few lines
To contact one, usually with a letter or note. I know you'll be busy enjoying yourself, but please, drop me a few lines on your trip. I'll drop you a few lines 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.