Английский Этимологический словарь - jerk (n.)

- 1935, "tedious and ineffectual person," Amer.Eng. carnival slang, perhaps from jerkwater town, where a steam locomotive had to take on boiler water from a trough between the rails because the train didn't stop there. This led 1890s to an adj. use of jerk as "inferior, insignificant." Probably also infl. by jerk off, slang for male masturbation (first recorded 1916). Sense in soda jerk attested from 1883.