From an Irish surname that was taken from \u00d3 Caiside and ultimately from the Gaelic word, cas. Debuted on the chart in 1981, possibly inspired by Shaun Cassidy, musician, actor, and TV producer who became a major teen idol with his late 1970s successful singles and role on the television series, "Hardy Boys." Used for boys, but more common for girls that follows a trend to use surnames as given names. Curly-headed. Surname.