Green’s Dictionary of Slang

to bring somone to orgasm.

1881		Loves of Venus 30: My prick was in fine state, she took it into her little mouth [...] and racked me off deliciously, never stopping [...] till she had drained the last drop.
1884		‘Experiences of a Cunt Philosopher’ in Randiana 71: She tongued me with a sweet solacing suck that almost drove me frantic [...] for she had begun to rack me off. [Ibid.] 126: She would have racked me off, but I restrained myself.
1898		‘Mary Suckit’ Yvonne 118: It was only when she became too agitated that he «racked her off», which always had the desired effect upon her nerves.