English, mainly US; generally a male name but, recently, female.

The origin of the name is unclear. ONC gives these possibilities:

     1. From the nickname of a folk hero 'Casey' Jones. His nickname derived from his birth place, Cayce in Kentucky.
     2. From an Irish Gaelic surname, with the meaning "watchful".
     3. Its use as a female name is probably a spelling variation of Cassie.