In life, everyone comes across someone who is mean on purpose, or someone who is self-centered, and thinks only of themselves, and/or someone who acts difficult for no particular reason and creates trouble. This someone is a jerk.
A jerk is an unlikable and obnoxious person, who acts mean, self-centered and disagreeable. Nobody likes a jerk, mainly because they tend to treat other people badly.
At times, jerk refers to a person who is foolish, or acts foolishly. It may even refer to someone who is naïve.
The synonyms of a jerk include numbskull, asshole, bastard, twat, knobhead, tosser, wanker, git, or dick.
In the traditional sense of the word, jerk refers to a sudden movement of the body, which is often uncontrolled. However, how this translated into the slang version of jerk meaning a mean, self-centered or disagreeable person is unknown.