ma'am
Standard use of the apostrophe is to fill in where letters or numbers are removed.
For example, "y'all" is spelled like so. That is also why you talk about the '90s as a fucked up decade, not the 90's - the latter indicated possessive, a while other use for apostrophes.
As for ma'am - it is shortened from madam. Webster tells us that the word came to English from Middle English, and before that from the Middle French
ma dame, literally "my lady."
Webster continues to define the current use of the word as, "used without a name as a form of respectful or polite address to a woman."
And thus concludes today's lesson.
Edit: Holy crap, this thread is moving fast - there were only a few replies when I started typing this ('course, I may have had the window open a while
)