Its source is Seissylt, an Old Welsh name meaning "The sixth child."
This boy's name is used in Polish.
Another possible source for this name (and a likely source of its European equivalents) is the Latin word caecus, meaning ''blind.'' This was the root of the ancient Roman family name Caecilius, ancestor to the modern girls' names Cecilia and Cecily, which are often considered female equivalents of Cecil.
However, Cecil was first used as an English surname by an aristocratic family of Welsh extraction. Thus, the English Cecil's relationship to the Latin term is probably a coincidence.