Welsh (Monmouthshire): from the Old Welsh personal name Seisyllt, sometimes wrongly said to derive from the Latin name Caecilius (see Sisley ). The most frequent modern spelling is a Renaissance attempt to connect the surname with Caecilius in the English pronunciation of its time. The Welsh name is more probably, but not definitely, from the Latin name Sextilius, a derivative of sextus ‘sixth’.

History: The great and powerful English Cecil family first came to prominence with David Cecil, a Monmouthshire gentleman who espoused the cause of Henry Tudor and came to court in London after the latter became king in 1485. His grandson William Cecil, Lord Burghley (1520–98), was Elizabeth I's chief adviser for 40 years, and his descendants have remained politically powerful and culturally influential in Britain ever since. They were originally minor Welsh gentry; their name is found in a variety of forms, including Sitsylt, Ceyssel, and Sisseld.