The expression "jerk-off" is much, much younger than the word "jerk" meaning a foolish or stupid person. Recall that a soda fountain attendant was also called a jerk (a soda jerk), a useage not found much today. And there are a lot of other meanings to jerk, including a treatment of pork or beef. Some of these go back a century or more, I would guess; but "jerk off " is probably 20th century.


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