[mam, mahm; unstressed muh m]
(in Britain) a term used in addressing the queen or a royal princess.
Origin of ma'am
First recorded in 1660–70; by contraction
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Ma'am is used as a title of respect, especially when addressing female royalty
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
also maam, 1660s, colloquial shortening of madam (q.v.). Formerly the ordinary respectful form of address to a married woman; later restricted to the queen and royal princesses or used by servants to their mistresses.
Online Etymology Dictionary, © 2010 Douglas Harper