With the growing popularity of Celia and Cecilia, this vintage canasta-playing nickname name could be due for a comeback.
Ceil had a few years of recorded popularity in the first decade of the twentieth century, peaking in 1905.
One prominent bearer was fashion designer Ceil Chapman (born Cecilia),who was a favorite of actresses from the forties to the sixties--she was one of Marilyn Monroe's favorite designers. She also provided the trousseau for young Elizabeth Taylor's first wedding.
Caveat: Might be misheard as Seal.