Green’s Dictionary of Slang

(US) wild, untameable.

1866		Weekly New Mexican 21 July 1/4: Then the Territory did not keep fast horses and other things, and go to bronco bailes and play whiskey poker [DA].
1887		F. Francis Jr Saddle and Mocassin 146: Sam’s too bronco; he gets all-fired mean sometimes when he’s full.
1898		F. Remington letter June in Splete (1988) 224: She wants to keep a rope on you – you have been broncho so long.
1910		Bulletin of Pan American Union 543: The Bacatete Mountains, the stronghold for ages of the wild or bronco Yaquis .
1920		‘Legend of Boastful Bill’ in J.A. Lomax Songs of the Cattle Trail 15: I’m a brocho twistin’ wonder on the fly.
1936		(ref. to 1920s) L. Duncan Over the Wall 213: You’ll go broncho whiffing so much of that damned merry [marijuana].
1947		Westerners’ Brand Book 75: He always felt that a man who made a false step wasn’t necessarily all bad or ‘broncho’ as he expressed it .