Its earliest use:

Originally Posted By: "the OED"

1 a. A form of respectful or polite address (substituted for the name) originally used by servants in speaking to their mistress, and by people generally in speaking to a woman of high rank; subsequently used with progressively extended application, and now, though no longer as frequently as in previous centuries, employed in addressing a woman of whatever rank or position; the female counterpart of sir


Later on it came to have "various derisive or opprobrious uses" like "An affected fine lady" and "A female brothel-keeper".

As for "why", well that's semantic change for you.

Last edited by goofy; 08/21/2011 8:41 PM.