Meaning

noun unacceptable behavior (especially ludicrously false statements)

originFrom buncombe, from “speaking to (or for) Buncombe County, North Carolina”, a county in North Carolina named for Edward Buncombe. In 1820, Felix Walker, who represented the county in the U.S. House of Representatives, rose to address the question of admitting Missouri as a free or slave state, his first attempt to speak on the subject after nearly a month of solid debate, right before the vote…

Etymology from Wiktionary, CC BY-SA 3.0