is many things, but all of which come from Celtic
As mentionned in the above writeups, it is an Irish name. Its feminine form is Ciannait and its pet form is Cianán. The Welsh version is Kian. As its Welsh form hints, it is pronounced "KEE-an", some (namely a friend of mine) prefer it pronounced as if it were "Sean."
There is also an Irish clan named Cian, also known as the Ciannachta, meaning "the children" or "the race of Cian".
In Celtic mythology, particularly referring to Tuatha De Dannan (the people of the goddess Danu), Cian (Cian Mac Cainte) was the son of Diancecht (the God of healing, medicine, regeneration, magic and silver-working) and who in turn was the father of Lugh with Ethniu/Ethlinn, daughter of Balor.
In the legends, he meets with the sons of Tuirenn -- Brian, Iuchar and Iucharba. His enemies approaching, he struck himself with his wand and turned himself into a boar. The Tuirenn brothers not able to distinguish between a real boar and a druidical boar, except for Brian who struck him and his brothers with his wand, changing them into hounds to pursue Cian. Eventually, they mortally wound him and Cian resumes his human shape before he dies. Lugh later finds his father's body with the help of the men of the Sidhe and vowed vengeance on his killers.
Encyclopaedia of the Celts
Shapeshifting in Celtic Myth