Diminutive of Cassius (Latin, "hollow") or Irish Cassidy ("curly-haired"). Used for both genders.
Cass arrives from two powerful sources that have never quite separated in the cultural imagination. The first is Cassandra, the Trojan princess of Greek mythology whom Apollo gave the gift of prophecy and then, when she refused his advances, cursed so that no one would believe her warnings. Cassandra foresaw the fall of Troy and was ignored.
Her name entered Western literature as the archetype of the unheeded prophet, and the clipped form Cass inherits a trace of that tragic authority. The second source is Cassius, the Roman family name borne by Gaius Cassius Longinus, one of Julius Caesar's assassins, and later made globally famous when Cassius Clay became Muhammad Ali. In practice, Cass as a given name or nickname has belonged primarily to women in English-speaking tradition, most memorably to Cass Elliot — 'Mama Cass' — the powerhouse vocalist of The Mamas & the Papas whose voice defined California harmony pop in the 1960s.
Her warmth, talent, and larger-than-life personality gave the name a particular kind of musical soul. The name also appears in Willa Cather's Nebraska fiction, in frontier-era records, and as a surname-turned-first-name in the American naming tradition that prizes short, punchy, gender-flexible options. Today Cass sits comfortably in the current fashion for clipped, androgynous names — alongside Kai, Jules, Quinn, and Sloane.
It works as a standalone name or as a nickname for Cassandra, Cassidy, or Cassius, giving it unusual flexibility. It is at once ancient in its roots and thoroughly contemporary in its feel.