Jackpot did not start as a general word for a huge prize. It began in poker in the 1800s. In some games, a player could not start the betting unless they held jacks or better, usually a pair of jacks or a stronger hand.
When someone finally had that hand and opened the betting, players put money into what was called the jack pot. Over time, the spelling shifted to the single word jackpot.
Later, the meaning widened. Instead of naming a specific poker pot, jackpot came to mean any especially large prize or payout.
- In poker: a jack pot was the pot opened by jacks or better.
- In modern gambling: a jackpot is the top prize on a slot machine.
- In everyday speech: people may say they hit the jackpot when they have a big success.
So the word moved from a very specific card game rule to a much broader idea, a sudden, valuable win.