The term 'Jack of the Nursery Rhyme' refers to the character Jack from the popular nursery rhyme 'Jack and Jill'. Jack is the protagonist of the rhyme, who goes up a hill to fetch a pail of water. The crossword solution for this term is 'SPRAT', which is a reference to the line in the nursery rhyme that reads 'Jack SPRAT could eat no fat'. This line is used to describe Jack's eating habits, as he is portrayed as being very thin.