The term 'Jack of Nursery Rhymes' is a reference to the character Jack from the English nursery rhyme 'Jack and Jill'. Jack is a young boy who goes up a hill with his pail of water, only to fall down and break his crown. The crossword solution for this term is 'SPRAT', which is a reference to another English nursery rhyme, 'There Was a Little SPRAT'. This nursery rhyme tells the story of a small SPRAT who was so small that he could fit through a keyhole.