Green’s Dictionary of Slang

bronco adj.

also broncho
[Sp. bronco, rough, esp. as applied to an untamed or half-tamed horse]

(US) wild, untameable.

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].
[US]F. Francis Jr Saddle and Mocassin 146: Sam’s too bronco; he gets all-fired mean sometimes when he’s full.
[US]F. Remington letter June in Splete (1988) 224: She wants to keep a rope on you – you have been broncho so long.
Bulletin of Pan American Union 543: The Bacatete Mountains, the stronghold for ages of the wild or bronco Yaquis .
[US] ‘Legend of Boastful Bill’ in J.A. Lomax Songs of the Cattle Trail 15: I’m a brocho twistin’ wonder on the fly.
[UK] (ref. to 1920s) L. Duncan Over the Wall 213: You’ll go broncho whiffing so much of that damned merry [marijuana].
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 .