Green’s Dictionary of Slang

(US) in a mad manner.

1918		‘Max Brand’ ‘Above the Law’ in Coll. Stories (1994) 65: You’re talkin’ loco!
1959		N. Nye Long Run (1983) 162: See him ride off with that trigger-loco Orso.
1972		R. Hogan Brandon’s Posse 122: You're talking loco. They'd never let me.
1999		W.J. Caunitz Chains of Command 94: The patron looked up at him again and slowly shook his head. ‘You're talking loco, Lieutenant’.
2007		S.M. Kaminsky Terror Town 204: Little guy, beard, wild hair, crazy eyes, talking loco.
2011		P. Plate Fogtown 33: What with all the trouble going down, the cops acting loco about the Brinks money, things were just too hot.