Green’s Dictionary of Slang

heart-on n.

[play on hard-on n. (1) + the stimulating effect on the heart]

(drugs) an inhalant.

1989–2003		R.O. Scott Gay Sl. Dict. 🌐 amyl nitrite: [...] Syn: heart on.
2001		ONDCP Street Terms 11: Heart-on — Inhalants.