Green’s Dictionary of Slang

ball off v.2

[balls n. (1)/ball v.3 ]

to masturbate; one of many sl. synons. for the SE verb that use ‘off’.

‘R. Blake’ Dict. Sexual Terms 12: ball off (vulg. sl.) To masturbate the male member [Simes:DLSS].
[US]B. Rodgers Queens’ Vernacular.