Green’s Dictionary of Slang

(US) Chicano street sl., linked to Mexican and gypsy patois.

1957		El Paso Herald (TX) 3 Oct. 17/3: The highly individual lnguage of the Pachcos, known as ‘calo’ .
1974		Fuentes & Lopez Barrio Lang. Dict. vi: The language of the Barrios is referred to as ‘Caló,’ and specially encircles the language of Chicanos in the United States.
1993		L. Rodríguez Always Running (1996) 165: Calo: Chicano street slang [...] linked to Gypsy patois of Spain and Mexico.