What does "I'll drop you a line" mean? Here's an extract from The catcher in the rye by J. D. Salinger; Then we shook hands. And all that crap. It made me feel sad as hell, though. "I'll drop you a line, sir. Take care of your grippe, now." "Good-by, boy." After I shut the door and started back to the living room, he yelled something at me, but I couldn't exactly hear him. I'm pretty sure he yelled "Good luck!" at me, I hope to hell not.