I won’t vouch for this story, found in Glover Moore’s The Missouri Controversy, although a quick check reveals internet sites repeating the substance:
As the debate over Missouri continued indefinitely, many of the speeches predictably became repetitious and boring. . . . Finally, when Representative Felix Walker of North Carolina rose to speak on February 25, [1820,] the House refused to listen to him. Walker represented the Buncombe County district. According to tradition, he stated that his constituents expected him to say something about Missouri and that he was bound to “make a speech for Buncombe.” In this manner, it is said, the word “bunkum” or “bunk” entered the American vocabulary.