Green’s Dictionary of Slang

1. (US) a lunatic.

1852		Alvarez Alvarez Papers in Carlisle Southwestern Dict. n.p.: Jerry Folger your friend has been trying to injure you his best by giving out [...] that you are a loco [DARE].
1918		G.E. Griffin Ballads of the Regiment 23: We are the lads who’ll smoke him out / On the trail of Loco Billy [i.e. Kaiser Wilhelm II] [DARE].
1924		C.E. Mulford Hopalong Cassidy Returns 15: ‘Got ’em yet?’ called the nester, carefully climbing a barbed wire fence. ‘Loco,’ grunted Hopalong. ‘Most nesters are,’ replied Red.
c.1943		‘Bill O. Lading’ You Chirped a Chinful!! n.p.: [heading] Locos.
1962		(con. 1953–7) L. Yablonsky Violent Gang (1967) 101: He was called ‘Loco’ for apparent reasons, and [...] had been under observation at Bellevue.
1981		J. Ellroy Brown’s Requiem 56: One loco made a lot of trouble.
1990		S. Morgan Homeboy 178: Tack another deuce on that loco’s time.

2. (US) madness.

1846		Durivage & Burnham Stray Subjects (1848) 130: Get out, with your d---d loco.
1907		S.E. White Arizona Nights 121: They had seen that white quartz with the gold stickin’ into it, and that’s the same as a dose of loco to miner gents.
1924		C.E. Mulford Hopalong Cassidy Returns 117: Yo’re full of loco!
1954		Hepster’s Dict. 7: Loco – The lack of intelligence.

3. (US) a car, a locomotive.

1926

J. Tully Jarnegan (1928) 164: Tell the Chink to bring the Loco right away.

4. (US teen gang) a Mexican-American gang member, thus fem. loca.

1988		J.D. Vigil Barrio Gangs 178: locos. Crazies, usually, in gang context, short for vatos locos.
1993		L. Rodríguez Always Running (1996) 83: I began high school a loco, with a heavy Pendleton shirt, sagging khaki pants.
1993		L. Rodríguez Always Running (1996) 105: She was [...] a real loca [...] she had the high teased hair, the short, tight skirts, the ‘raccoon’ style makeup.

5. (US drugs) marijuana.

1969		R.R. Lingeman Drugs from A to Z (1970).
1971		E.E. Landy Underground Dict. (1972).
2001		ONDCP Street Terms 14: Loco (Spanish) — Marijuana.