Had to go to the comments in the linked post to find the reference to the soda jerk. And "jerking" was apparently pulling on the (seltzer) tap.

I suppose if "soda jerk" was thought to be a menial job (reminded of one of the threads in the film, "The Best Years of Our Lives" (1946)) ... then the original meaning of "jerk" as a loser might have come from that.