What is an "M.C."?
In order to determine the identity of the "wife of an M.C.," it is first necessary to figure out what Lincoln meant by "M.C." There are several possibilities: "master of ceremonies," "member of congress," "master chef," and "Marine Corps" might come to mind today. Or perhaps, "M.C." refers to some other title that is no longer in current use.
In order to determine what Lincoln meant, the Kalamazoo Abraham Lincoln Institute's Tom George searched for the use of the phrase "M.C." throughout Lincoln's other letters and writings. He found 45 examples where Lincoln used the phrase "M.C." In eight of these instances, the initials "M.C." were part of a proper name, like "M.C. Meigs" (the quartermaster general of the Union army). The remaining 37 times, "M.C." was used as an abbreviation for "member of Congress." Never was "M.C." used to abbreviate "master of ceremonies," "Marine Corps," "master chef," or any other title.
So, when Lincoln wrote, "the wife of an M.C.," he was referring to the wife of a member of Congress.