Green’s Dictionary of Slang

an act of masturbation.

1972		B. Rodgers Queens’ Vernacular 116: mutual masturbation by two men. [...] hand job masturbation performed by one man upon another [...] JO.
1989		Chapple & Talbot Burning Desires 303: The exposed nature of J/O parties also made it virtually impossible for overheated celebrants to break the rules and conjoin their bodies.
2003		K. Cage Gayle 77/1: JO (abbr.) jack off, masturbate • JO club gathering of men for the purpose of masturbating together (cf. circle jerk) – these clubs became popular in the late 1980s as an alternative to unsafe sexual practices such as fellatio and intercourse.