Short form of Cassius or Casper, from Latin meaning "hollow" or Dutch/Persian meaning "treasurer."
Cas operates as an elegant crossroads name — short enough to stand fully alone, yet serving as a familiar form for a remarkable range of longer names: Cassius, Casimir, Caspar (or Casper), Cassandra, and Castiel, among others. The Latin Cassius, borne by the Roman general Gaius Cassius Longinus — one of the principal conspirators in the assassination of Julius Caesar — carries connotations of political shrewdness, lending the root a sharp historical edge. Muhammad Ali famously reclaimed the name by birth as Cassius Clay before his conversion.
Casimir, meanwhile, is a proud Slavic name meaning 'proclaimer of peace,' borne by kings of Poland and the patron saint of Poland and Lithuania. Caspar, one of the traditional names given to the three Magi of Christian tradition, has been carried since the medieval period across Germany and the Netherlands, giving Cas an unexpected connection to the nativity story. Cassandra, the Trojan princess cursed by Apollo to prophesy truly but never be believed, gives the name a poetic, tragic dimension when used as a feminine source.
In the twenty-first century, Cas gained particular cultural currency through the television series 'Supernatural,' in which the angel Castiel — called Cas — became one of the show's most beloved characters over fifteen seasons, introducing the name to an enormous fanbase. The result is a name that is simultaneously ancient and genuinely current, minimal in form but maximal in reference.