"Bunk" is an American (slang?) expression meaning nonsense.
To "debunk" means to remove nonsense.
This being said, the term "debunk" is used in isolation--no one ever would refer to "bunking a myth."
Other examples of words that act like this are
- decapitated: if someone has their head, they're not "capitated"
- invincible: someone who can be defeated is not "vincible"
(Source Online Etymology Dictionary http://www.etymonline.com/index.php?term=bunk&allowed_in_frame=0 )