Cassidy is a given name derived from an Irish surname and ultimately from the Gaelic given name Caiside, meaning "clever" or "curly-haired." The name Caiside comes from the Gaelic word element cas, according to "Behindthename.com" Cassidy was the 244th most popular name for girls born in the United States in 2009.
Transferred use of the surname that is derived from the Irish surname O'Caiside which is of uncertain meaning. One historian believes it is derived from the Gaelic elements cas (a twisted lock of hair; ingenious, clever, sly) and the personal suffix -idhe: hence, "clever one, one with the twisted locks."
