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. 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.

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.
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.

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.