Where did the word Jackpot originate?

Jackpot goes back to draw poker in which the stakes are allowed to accumulate in the "pot" until a player possesses a pair of jacks or better, at which point that player can begin play. 

Since the players bet on each hand until someone can open, the pot can grow quite large, which led to such pots being called jackpots. There is evidence of jackpot having been used in this sense as early as the 1880s. 

The popularity of poker led eventually to the broader use of the word to mean "an impressive and often unexpected success or reward." The phrase "hit the jackpot," meaning "to become notably and unexpectedly successful," appears in the 1940's.