Green’s Dictionary of Slang

(US drugs) to inhale a narcotic.

1961		Rigney & Smith Real Bohemia xx: To take nasally is to ‘horn,’ ‘sniff,’ or ‘snort’.
1978		H. Selby Jr Requiem for a Dream (1987) 32: So Tiny horns a little just to be cool [...] and he gets wasted jim. [Ibid.] 115: Im not gonta waste no good dope by hornin it man.
1990		S. Morgan Homeboy 49: Horned the last [i.e. portion of heroin] just an hour ago.
2001		ONDCP Street Terms 12: Horn — To inhale cocaine; [...] to inhale a drug.