There seems to be two meanings for this idiom, both dependent on the same metaphoric setting and action ”” but while one is mostly positive, the other, not so much. The first connotes a situation in which a person does something (whistling, maybe?) to make of show ”“ to others, or even more commonly, to oneself ”” of bravery, or at least nonchalance, in the face of danger or difficulties.
The second meaning describes an individual who is genuinely confident and cheerful while in pursuit of a course of action at the same time blithely oblivious to the real risks involved ”“ i.e., clueless.