Over the years, a number of theories have been floated as to why Mrs. March is called Marmee by her daughters. However, the most likely explanation is that it's how the word “Mommy” is pronounced in parts of Massachusetts, which is where the story is set.

It's generally believed that this is an expression commonly used by Louisa May Alcott herself, and so it's unsurprising that it should've found its way into Little Women. If one takes into account Alcott's usage and the accent of Eastern Massachusetts, then the precise origins of “Marmee” are not that difficult to uncover.

In any case, Marmee is clearly a term of affection that indicates the closeness of the relationship between Mrs. March and her daughters. In those days, it would've been quite common for children to refer to their parents as “Mother” and “Father,” much more formal modes of address. And although the March girls do address Marmee as “mother” on a number of occasions in the book, it's notable that the more informal “Marmee” is much more commonly used.

One could say that the two terms, as used by the March girls, indicate a combination of affection and respect entirely appropriate for such a remarkable woman and mother.