Green’s Dictionary of Slang

(US Und.) a rapist, by ext. any sex criminal.

1949		N. Algren Man with the Golden Arm 197: Ancient pious perverts and old blown parolees, rapoes and record-men.
1953		T. Runyon In For Life 22: The few rapos I’ve talked to who raped adult women.
1969		B. Jackson Thief’s Primer 143: Your rapos, they get ahold of the Bible and they start going to church, and they stay there.
1978		R. Caron Go-Boy! 283: Fourteen diddlers, rapos and stool pigeons were tied with chains in a circle in the centre of the dome.
1985		Southern Illinoisan (Carbondale, IL) 7 Apr. 4/1: Prison Slang [...] Rapo: Someone who commits a sex crime.
1986		J. Ellroy Silent Terror 141: ‘[W]e typed the rape-o sonuvabitch we're looking for from his semen and he's O positive’.
1992		Bentley & Corbett Prison Sl. 37: Rape-O An inmate who is in prison for rape.
2003		J. Ellroy ‘Stephanie’ in Destination: Morgue! (2004) 57: Rape-os, rape-o killers, bondage rape-os. Burglar rape-os, knife rape-os, gun rape-os, child rape-os, girl rape-os, woman rape-os, old lady rape-os.
2021		J. Ellroy Widespread Panic 50: Dig: three hot-prowl/rape-o/459 men.