If we pronounce the title Mrs. as “missus,” why is there an r in it? Despite its pronunciation, the abbreviation Mrs. is derived from the title mistress, which accounts for that confusing extra letter. Mistress is the counterpart of master, which—you guessed it—is abbreviated to Mr. (Of course, English speakers now pronounce the title Mr. as “mister.”)
While mistress may have distasteful connotations today, in the mid-18th century the title referred to a woman of economic or social capital. Mrs. was an honorific: a woman referred to as Mrs. generally had servants or was part of an upper social echelon. Most notably, the title Mrs. did not signify that a woman was married, just like Mr. today. In fact, Samuel Johnson’s dictionary of 1755 offers six definitions for the word mistress, which range from the respectful (“a woman who governs” or “a woman skilled in anything”) to the ironic (“a term of contemptuous address” or “a whore or concubine”), but no definition mentions marital status.
The use of Mrs. to refer to a married woman is linked to the history of another title: Miss. Miss became a popular title in the late 18th century and specifically referred to an unmarried woman (often a schoolteacher) of a high social status. (Originally, Miss was actually a title for young girls, while Master was the title for boys.) This, according to scholar Amy Erickson, caused a shift in the use of Mrs. to signify a married woman in the late 18th century and still informs our use of the title Mrs. today.
How the pronunciation of mistress turned to “missus” is somewhat unclear. Erickson cites John Walker’s A Critical Pronouncing Dictionary, and Expositor of the English Language from 1828: “The same haste and necessity of dispatch, which has corrupted Master into Mister, has, when a title of civility only, contracted Mistress into Missis.” The change in pronunciation was essentially a colloquial and utilitarian shortening, and by the tail end of the 18th century, this pronunciation was the preferred one.