It was a drinking game to keep people from driving drunk. I first heard the term in the late 1960รขยยs.
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.
After saying the name of his favorite beer, he took forever to get his 3 knocks in; thereby being declared The Fartknocker.
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