It was a drinking game to keep people from driving
drunk. I first heard the term in the late 1960’
s.
Imagine a group of men in a bar, getting inebriated and passing
gas. When a guy passed gas, the men would have to state the name of their favorite
beer, followed by 3 quick knocks. The one with the slowest response time was labeled the (slowest) fart knocker; and accordingly had to stick around (as in not leave yet), as that was the most impaired person.