What is "fitzwilly"

Fitzwilly is a 1967 film directed by Delbert Mann, based on Poyntz Tyler's 1960 novel A Garden of Cucumbers and adapted for the screen by Isobel Lennart. Its title refers to the nickname of its protagonist, Claude Fitzwilliam, an unusually intelligent and highly educated mastermind of a butler played by Dick Van Dyke (the novel's title refers to Isaiah 1:8). The film co-stars Barbara Feldon in her first feature-film role.