Question: In season 1, episode 2 of Samurai Jack, Jack comes out of the time portal in the future and lands in a city with flying cards and other machinery he does not recognize. After some action sequences Jack lands to safety next to some onlookers who cheer him and say things like

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> Yo, **Jack**, that was some awesome show
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> I have never seen a punk with moves like that, **Jack**
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> Word! **Jack** was all ricochetic and jumpadelic!
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Why do they call him **Jack** though? Jack would later go on to take this name, but it is unclear why the onlookers would call him Jack before this though.

Answer: 
Why do you call someone 'Dude' or a friend of yours 'Buddy' ?

That is basically what happened there. We can see with the onlookers character design that they are depicted as teenagers and 'Jack' probably has the same meaning as 'Dude' for extraterrestrial teenagers in the future.
*Jack* is US slang (seemingly out of date since the 90's) for referring to a man, similar to *dude*, *man*, *bro* etc:

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> **6** informal Used to typify an ordinary man.
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> > ‘he had that world-weary look of the working Jack who'd seen everything’
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> **6.1** *US* **Used as a form of address to a man whose name is not known.**
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* <https://en.oxforddictionaries.com/definition/jack>