Cassidy is the anglicized form of the Irish surname Ó Caiside and means "descendant of Caiside", a byname that is usually explained as "curly-haired". It became a first name in the United States in the 1970s, helped by the Grateful Dead song Cassidy and by the popularity of singer David Cassidy. Cassidy ranked number 107 in 2000, just outside the top 100, and has gradually eased to number 541 among girls' names in 2025.