Its source is Cathasaigh, a Gaelic name meaning "Alert."
The name Kacy ranked 2720th in popularity for females of all ages in a sample of the 1990 US Census.
Though this name appears on the 1990 U.S. Census lists, it is used by only a small percentage of the general population.
The popularity of this name owes a great deal to American railroad engineer Casey Jones (1863-1900), who sacrificed his life to save his passengers in an incident memorialized in legend and song. He was born Johnathan Luther Jones, and derived his famous nickname from his hometown of Cayce, Kentucky.