The term jack off is a slang term that refers to penile masturbation. The term may stem from the use of the word "jack," a device used to lift heavy objects, often a car. A car is said to be "jacked up," when it is lifted and, much like male masturbation, doing so involves a repetitive, one-armed motion.
The term is also used to refer to someone who is being idle or unproductive.
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