Although there is some controversy surrounding the origins of "Jack and Jill," one common suggestion is that Jack and Jill are King Louis XVI and Marie Antoinette, respectively, who were beheaded in the 18th century. Thus, King Louis XVI literally broke his crown, as did Marie Antoinette shortly after. The actual origin of the story is complicated, however, by possible evidence of it dating as far back as the 14th century in the works of Shakespeare.
Another possible origin comes from the 17th century with King Charles I reducing the volumes of liquid measures of the half- and quarter-pints, also known as jacks and gills, respectively. When the jack "fell down," the gill subsequently dropped.
A third suggestion of origin of the nursery rhyme comes from the 13th-century Icelandic story Gylfaginning, about Hjuki and Bil, who the god Mani took up to the moon as they were fetching water from the well. In Scandinavian folklore, the shapes on the surface of the moon are said to be two children carrying a pail of water.
Another origin story comes from 17th-century Kilmersdon, England, where the rhyme allegedly originated, in which Jack and Jill were a couple who habitually snuck up the hill to be alone together. According to this story, the man, Jack, fell down, hit his head and died. The woman, Jill, had become pregnant and died shortly after during childbirth. In 2000, the town erected a plaque with the rhyme engraved to commemorate this historic connection.