Green’s Dictionary of Slang

1. (US black) any Mexican, Puerto Rican or other Latin person (cf. vato n.).

1971		E.E. Landy Underground Dict. (1972) 29: bato [...] Male; guy; male who is tough; gang member.
1971		F. Hilaire Thanatos 151: You make eet weeth thees bato?
1985		‘Jennifer Blowdryer’ Modern English 57: bato (n): (vah-to) A guy, a dude.

2. (US Hisp.) a general term of address.

1971		F. Hilaire Thanatos 150: Hey mahn. You, broad; you, bato.
1972		B. Rodgers Queens’ Vernacular.
1992		Bentley & Corbett Prison Sl. 51: Bato The English equivalent is dude. ‘Hey bato, what’s happening.’.