Green’s Dictionary of Slang

1. a member of a Mexican teen gang; esp. one with a reputation for extra violence, poise, courage and other attributes of street life.

1968		S. Steiner La Raza 119: Vato Loco is untranslatable, meaning something like ‘crazy mixed-up kids’.
1970		C.G. Laird Lang. in America 392: In Calo terminology a human being is either an escuadra, a law-abiding person, or he is a vato loco, one who spends a reasonable amount of time in jail for stealing and handling dope.
1985		O.Z. Acosta in Eisen et al. Unknown Calif. 304: Your friends know you to be a vato loco, a crazy guy, and they call you ‘ese,’ or ‘vato’ [...] There is no school for a vato loco. There is no job in sight.
1993		L. Rodríguez Always Running (1996) 6: I jumped over peeling fences, fleeing vatos locos, the police or my own shadow.
1998		Austin Chronicle (TX) 25 Nov. n.p.: Skillfully adapting with ease to such characters as ‘Joe’, the city bus driver who’s seen everything, or ‘Chuy’ el bato loco, breaking into a house.
2002		Casa Zapata 🌐 At the top of the mural, Michelangelo’s Adam has been transformed into a Chicano bato loco, cigarette in hand, ashes ready to fall, complete with a tear tattoo and pachuco cross.
2023		(con. 1962) J. Ellroy Enchanters 355: He wore khakis and a Sir Guy shirt. He dressed classic vato loco.

2. an affectionate nickname for a ‘crazy’ (fellow) Spanish-American.

1971		E.E. Landy Underground Dict. (1972) 29: bato loco (Spanish) n. Guy who will do anything; loco means crazy.
1980		E. Folb Runnin’ Down Some Lines 229: bato/vato loco Crazy-acting male (usually refers to a Latino in general and a Chicano in particular).
1992		Bentley & Corbett Prison Sl. 51: Bato Loco A crazy dude. Normally used in a joking, friendly manner.
2002		Azteca.net 🌐 My uncles Max and Nacho along with several of their Dog Town comrades offered nothing but kudos for my would-be father, lionizing him as a bato loco.