I believe this comes from soda jerks: the guys who jerk the soda taps and make you an egg cream or a chocolate coke. I should probably say "made," as it's a job

I believe this comes from soda jerks: the guys who jerk the soda taps and make you an egg cream or a chocolate coke. I should probably say "made," as it's a job that was eliminated by soda machines. A lot of restaurants even have customers "Jerk" their own soda, though now you just press a cup against a thingie.