The name Fitzwilliam is a boy's name of English origin meaning "son of William". Fitzwilliam and is often added to lists like Literary Names for Boys and discussed in our forums with posts like "Middle name for a First name!".
From the experts:
The Christian name of the dashing Mr. Darcy in Pride and Prejudice— it was his mother's maiden name— is just one of several Fitz names, including Fitzroy, Fitzgerald, Fitzpatrick, that could be used to honor a dad named William, Roy, Gerald or Patrick.