M'lady is a diminuitive of My Lady which it'self is a diminuitive of My Ladyship
Made socialy acceptable and stereotypical of Pearly King and Queen types and the lead Dick Van Dyke from the film Mary Poppins and others in the genre. The Cockney accent became a marketable and charming odd ball accent valued by America's film goers and thereby promoted worldwide.
It was cheeky, rough and ready , lower working class and cute especially to American women. I theorise that there were many, many WWII brides from Britain living with their Yank husbands in the USA who themselves promoted such films because they missed the accents and sound of the British working class from whence they came!
Was also a sneer when used in certain tones and timing! A kind of "Yes M'Lady!!" in a sneering nature denotes that the servant had had enough of the boss!! The staff member would have definately been let go or sacked as a result of said slip!
Also used by male to address a female when said couple are very much in love.
"My heart and soul is yours M'Lady"
My Lady, My woman, My mate, My lover
Romantic language, poetic and archaic.
Term still used, mostly by song writers.