Why “off ” in jerk off, jack off, get off?
The off in those particular verb phrases is used as an adverb. The meaning I'll take from wikipedia:
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In a direction away from the speaker or object.
He drove off in a cloud of smoke.
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Into a state of non-operation; into a state of non-existence.
Please switch off the light when you leave.
So in the case of jerk off we're speaking of a jerking motion away from the ahem "object". Jack off is just a bowdlerized version of the phrase. Get off usually means to get away from (so that you're no longer on top of) something or someone, or more metaphorically to experience excitement or orgasm. In those instances, it's evoking really the "out of body" experience.
You could also interpret the meaning of off in get off to be sense 2: you're "going until you're finished."