Green’s Dictionary of Slang

(US) a veteran of gang life.

1981		J. Wambaugh Glitter Dome (1982) 119: When he was old enough he hoped to be a cherry, then a cutdown, then finally, after he’d been shot and stabbed ten times and was too old to fight, a veterano.
1998		Austin American-Statesman (TX) 14 Oct. 9/4: Texas Prison Gangs Slang [...] Veterano: Veteran gang member.