"Mrs." is actually an abbreviation for the word "mistress."

"Mistress" is the feminine form of "mister," which originally meant "master." Over the years people garbled the name for the married female, and the word "mistress" took on the meaning of an illicit lover, something far different than its original connotation. Therefore, married women ceased to spell out the full word, leaving "Mrs." as the far more respectable abbreviation!

(source)