I subscribe to a daily trivia facts list and thought todays might be of interest. Here it is:

Question: Why is a big gambling payoff called a jackpot?

The Answer:

It's a safe bet that most people don't know Jack about what
kind of pot this expression refers to. Ordinarily I might play
my cards close to my vest and tease you a while about what it
means, but I'm feeling pretty flush today, so I'll just play
it straight and deal.

The game is poker, folks--draw poker, to be exact. You need a
pair of jacks or better to open, but you ante-up regardless.
The longer we play without someone holding the requisite
cards, the more money there will be in the pot. Eventually
someone will draw the cards that will win the...you-know-what.
(What are you grinning about, pardner? Just what cards are you
holding?)

By extension, the payoff at slot machines is also commonly
called the jackpot.