by 1917, American English, abbreviation of Masters of Business Administration, an educational degree attested by that name by 1912.

affixed to the name of a medical doctor, by 1723, an abbreviation of Latin Medicinæ Doctor "doctor of medicine."

1917, abbreviation of military police, which is recorded from 1827. By 1809 as an abbreviation of Member of Parliament.

childish or colloquial shortening of mamma, by 1823. "Also applied colloq. to a middle-aged or elderly woman, esp. one in authority" [OED].

also maam, 1660s, colloquial shortening of madam (q.v.). At one time 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. In U.S., used especially in answers, after yes or no.

fem. proper name, shortening of Amabell, Amabillia (c. 1200), fem. formations from Latin amabilis "loving; lovable; pleasant, attractive," from amare "to love" (see Amy). In the U.S. it enjoyed its greatest popularity as a given name for girl babies from c. 1884 to 1895.