Jack was a poor and innocent boy. One day his mother sent him to the market to sell their last and only possession, a cow, but along the way, Jack met a stranger who offered to trade it for five "magic beans". Thrilled at the prospect of owning magic beans Jack made the deal without hesitation. But his mother turned out to be less than thrilled when he arrived back home, she threw the beans straight out of the window and left Jack without dinner. Overnight however, the seeds grew into a gigantic beanstalk to the sky. Jack was a curious boy so he decided to climb the plant and arrived in a land high up in the clouds, which was the home of the giant who killed his father. Inside his castle, he stole some gold coins, the giant noticed his presence but Jack manages to escape whit the giant's wife help. Back home the boy and his mother celebrated their newfound fortune, but their luck did not last, and Jack climbed the beanstalk once more. This time he stole a hen which laid golden eggs. Again he was saved by the giant's wife. He went down the ladder and showed the chicken to his mother, and the two lived happily on the proceedings from the hen's eggs. But one day Jack grew bored, and he resolved to climb the beanstalk a third time. This time, he stole from the giant a magical harp that called out to the giant for help. The giant chased Jack down the beanstalk, but luckily the boy got to the ground before he did. Jack immediately chopped it down, making the giant fall.

In One Piece appears the Giant Jack, is a beanstalk that communicates Skypiea with the God's Shrine. Giant Jack's name, as you already deduce, comes from the main character of this story. The giants could be related with the shadows of the inhabitants of Skypiea that sometimes are reflected in the sky.