If 'Mr' is the abbreviation for 'Mister', what is 'Mrs' short for?

Mrs. originated as a contraction of the honorific Mistress, the feminine of Mister, or Master, which was originally applied to both married and unmarried women. The split into Mrs. for married women from Ms. and Miss began during the 17th century.
https://en.wikipedia.org/wiki/Mrs.

Direct quote from Oxford Dictionaries site:

"'Mistress' used to be the title prefixed to the name of a married woman, a usage dating back to the 15th century, as these early examples in the Oxford English Dictionary (OED) show:

1463 in S. Tymms Wills & Inventories Bury St. Edmunds (1850) 36, I ... be qwethe to my maistresse Clopton a spoon of berell.

1471 J. Paston in Paston Lett. & Papers (2004) I. 443 Iff it come to Mestresse Elysabe? Hyggens ... sche schall comveye it to me. "

https://www.dictionary.com/e/mr-mrs/