The titles "Mr." and "Mrs." are so commonly used in every day interactions that no one really thinks twice about them.
Obviously, "Mr." stands for "Mister" and "Mrs." for "Missus." But, doesn't "Missus" sound so awkward written down in full? It turns out that everyone used to think so as well.
According to Mental Floss, the original meaning for "Mrs." was Mistress. But it's not what you think. If you're an avid reader of the classics (i.e. Shakespeare, Dickens, the Brontë sisters), you know that the definition doesn't have the same meaning it does now.
The Mistress wasn't "the other woman" of a cheating husband or a sexy dominatrix (there are two kinds of people in this world); she was the female head of a household or a governess who looked after the children. It was also a title given to married women who were often called the mistress of their household. In short, it was the female version of "Master."
Speaking of which, back then, "Mr." was used for "Master" but was changed later on to "Mister" (the word did exist to mean an occupation or a skill).
Mental Floss said, "Eventually, the title form took on a contracted, 'r'-less pronunciation, and by the end of the 18th century 'missus' was the most acceptable way to say it."