Likely from an Irish surname related to Callahan, traditionally interpreted as 'bright-headed' or 'descendant of Ceallach.'
Calihan is a melodic Americanization of Callahan, itself derived from the Irish Gaelic surname Ó Ceallacháin — meaning "descendant of Ceallachán," a personal name whose roots are debated between "lover of churches" (from "ceall," church) and a diminutive form of a word suggesting brightness or strife. The Callahan sept was prominent in County Cork, and the name traveled to America in great waves with the Irish diaspora of the 19th century, eventually becoming one of the more recognizable Irish-American surnames — most iconically attached to Harry Callahan, Clint Eastwood's uncompromising detective in the Dirty Harry films.
The shift from Callahan to Calihan as a given name reflects a broader trend in American naming culture: the reclamation of Irish and Celtic surnames as first names, a pattern that includes Brennan, Sullivan, and Cassidy. The softer "Calihan" spelling smooths the surname's rough edges, making it feel more like a given name while preserving its Gaelic heritage. It sits comfortably alongside Callum, Calen, and Cillian in the modern landscape of Celtic-adjacent names for boys.
As a given name, Calihan carries a warm duality: the old-world charm of County Cork alongside the frontier confidence of an American surname. It has no famous individual bearers as a first name yet, which gives it a rare quality in contemporary naming — the feeling of discovery, of a name with substance and roots that nevertheless feels genuinely new.