To get to the origin of this word, we need to take a look at Asheville, North Carolina, which is in the county of Buncombe. From 1817 to 1823, Buncombe sent Felix Walker to Washington to serve in the House of Representatives. On February 25, 1820, while a debate was taking place on the Missouri Compromise, Felix Walker motioned that he would like to speak. After he took the floor, he said, "I shall not be speaking to the House, but to Buncombe." He admitted later that he merely wanted his constituents to see his name in the paper, and know that he was doing things in Congress.
As news spread of Walker's speech, and the reason for it, "a speech for Buncombe" came to signify meaningless, frivolous, repetitious, and unnecessary speech. Eventually "bunkum" — the phonetic spelling of Buncombe — took hold, and in due time that was shortened to simply "bunk."
In 1868, Mark Twain used "buncombe" in referring to Gilbert H. Bates of Wisconsin.
"Do you know that that idiot is aspiring to the Presidency of the United States? He honestly is. He said in a farewell speech on shipboard, as he left New York — a speech slobbering adulation and nauseating buncombe over half a dozen Irishmen out of business, that in due time he would be the People's President."