horse (noun) — sense 1

Riccio nodded toward a boy across the floor and said, “See that kid? He’s on dope.” The boy was standing against a wall, staring vacantly at the dancers, his face fixed in a gentle, faraway smile. Every few seconds, he would wipe his nose with the back of his hand.
“Man, that Jo-Jo!” Benny said. “He’s stoned all the time.”
“What’s he on—horse? Riccio asked, meaning heroin.
“Who knows with that creep?” Benny said. I asked Benny if any special kind of boy went in for dope.
“The creeps,” he said. “You know, the goofballs.” He searched for a word. “The weak kids. Like Jo-Jo. There ain’t nothing the guys can’t do to him. Last week, we took his pants off and made him run right in the middle of the street without them.”