Bunk can be a narrow bed or, mainly in the United States, a verb which means to room with another person. It can also be a mass noun for foolishness or nonsense. This is a shortened version of the word bunkum, which has the same meaning. Side note: bunkum may also be spelled buncombe because the term originated from a politician in Buncombe, North Carolina.
Outside of the United States, there is a phrase do a bunk that means to hastily leave or escape.
Debunk is a verb that means to prove an idea or belief false. It is usually used with things like myths, legends, or even perceptions of people.
The noun form is debunker or debunkery.
The word does stem from bunk. Debunk was used in a work of fiction as a term for removing the bunk from topics or thoughts.
Examples
“This whole notion of an earnings recession is bunk, as my grandmother used to say,” he told CNBC’s “Squawk Box.” [CNBC]
The April 30 editorial “Burrito bunkum” excoriated Chipotle’s decision to ban food made with genetically modified organisms (GMOs) from its menu. [The Washington Post]
Robert is a deeply innocent man in an ancient land that has emerged from a period of immense trauma, and while his innocence is both alluring and dangerous, his rationalist European mentality is woefully unequipped to read a culture that places such great store on the irrational – on signs, omens, dreams, on the sixth sense, on ghosts and spirits; in short, everything that the European Enlightenment taught the western world to deride and debunk. [The Guardian]