The Penny Dropped

What is the meaning of this pharse and where did it originate from? "The Penny Dropped"

It means that (finally) you have understood something, you have made the connection, "light has dawned". The image is of a stuck slot machine; you put in your penny but nothing happens, and you have to thump the machine and rattle it till "the penny drops" and out pops your ticket, your chocolate bar, whatever.