Green’s Dictionary of Slang

1. as term of address.

1928

J. Callahan Man’s Grim Justice 139: Play ’em [i.e. ‘cows’] once in a while, broncho, but don’t tie up with ’em.

2. (US Und., also cat’s kid) a spy sent out by safe-blowers to locate potential robberies.

1926

‘Und. and Its Vernacular’ in Clues mag. 158–62: broncho Spy sent out by safe-blowers to locate a job [...] cat’s kid. broncho.

3. (also bronco-boy) a young man, a novice in the gay world and thus somewhat rough; thus bronco-buster n., an older man who favours sex with young/underage boys.

1927		‘Vagabond Argot’ in AS ii 387/2: Here and there [...] men can be observed traveling with boys and living with them in the relationship politely known as Socratic love. The man is called a wolf or jocker; the boy has many names,—punk, gazooney, guntzel and bronc.
1933		Ersine Und. and Prison Sl. 21: bronc, bronco, n. A homosexual boy, a punk.
1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 5: bronco (n.): Young male, new to homosexual practices, who is normal, rough, and at times intractable to the advances of the homosexual.
1972		B. Rodgers Queens’ Vernacular 45: Elderly man with a voracious appetite for young roosters [...] Syn: bronco-buster [ibid.] 155: The sexually dominant prisoner..is called anus bandit, bronc(o)-buster,..gut butcher [etc].
1972		R.A. Wilson Playboy’s Book of Forbidden Words 50: Bronco. A young male recently initiated into homosexual activity who may be difficult to deal with.
1978		‘S. West’ All Day Sucker 183: [I] started ordering them by the dozen. A new notebook for each tomcathouse I managed, a new roster of willing-to-serve-you broncos [Simes:DLSS].
1997		Mantalk (N.J.) Nov. 124/2: And since I’m a cowboy, I do know how to bust a bronco boy and ride that sweet mouth and tight ass [Simes:DLSS].

4. (US prison, also bronk) the effeminate companion/lover of a ‘masculine’ prison homosexual.

1953		T. Runyon In For Life 99: I’ve seen the patient papas carrying sacks of canteen stuff for their boys — also known as bronchos, gunsels, kazoonies, kids, brats, and, mostly, punks.
1960, 1975		Wentworth & Flexner DAS 64/1: bronc bronk [...] A catamite.