I believe the term came from footage shot on the frontlines of Vietnam where an infantryman actually used his shotgun barrel to blow smoke into the mouth of another infantryman. I've seen this footage numerous times and it's most likely on YouTube if you're so inclined.
Man that takes me back 40 years when I smoked, its a inefficient way to smoke, you placed a lit joint backwards in your mouth, and exhaled. This was slightly more than dangerous with the burning ash inside your mouth, but well worth the effort if your smoking companion was the opposite sex. Hargh!!