A quick, sudden movement, or slang for a foolish or unpleasant person.
The word for the movement comes from an American frontier term, possibly from 'jerk' relating to 'jerking' or sudden pulling. When used for a person, it's American slang dating to the 1930s.
The word 'jerk' as an insult is surprisingly modern—it only became slang for 'annoying person' in the 1930s in America, so it's younger than your great-grandparents, even though the physical motion 'jerk' is much older.
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