Although this is a grammatically masculine adjective in French, it is given to girls as well as boys in Britain and the Netherlands. In America it is more exclusively masculine. It is not commonly used as a name in France itself.
Due to the popularity of the saint, the name became common in the Christian world during the Middle Ages. The Normans brought it to England, where it was commonly spelled Cecily — the Latinate form Cecilia came into use in the 18th century.
This is also the name of an Irish county, which was itself probably derived from Irish clár meaning "plank, level surface".
In England the name was popularized when Princess Anne gave it to her daughter in 1981. Use of the name may also be influenced by the trendy Spanish clothing retailer Zara.