The spelling of the name "William Duff" is based on the English language phonetics. In IPA transcription, the name would be pronounced as /ˈwɪljəm dʌf/. The "w" at the beginning of the name represents the voiced labial-velar approximant, while the double "l" in "William" stands for the voiced alveolar lateral approximant. The "d" and "f" in "Duff" represent the voiced alveolar stop and voiceless labiodental fricative respectively. The written spelling accurately reflects the pronunciation of the name when spoken in English.
The term "William Duff" does not have a defined etymology as it is a combination of a first name "William" and a last name "Duff". "William" is of Germanic origin and comes from the Old High German name "Willahelm", which means "will/desire" and "helmet/protection". On the other hand, "Duff" is a Scottish surname derived from the Gaelic word "dubh", meaning "black". It is likely that the combination "William Duff" is a personal or family name without a specific etymology beyond the origins of the individual components.