Origin of Phrases

[karmenite]

What is the origin of "Drop me a line"?

[Quizmonster]

The earliest recorded use of the phrase dates back to the 1760s, so it has been around for quite a while. The idea behind it is that a letter, when pushed through a letter-box, falls to the floor or at least the bottom of the mail-box inside.
'Line' is just an example of the figure of speech, synecdoche...using a part for the whole. That is, a letter consists of quite a few lines, but one refers to that in the singular. (The same sort of thing is seen in 'All hands on deck!' when the Captain clearly expects whole bodies to appear, not just hands!)